This is supposed to be when the fun starts

I'll take a different tack than all the other replies so far (which are valid both as descriptors of the structural factors behind what you're encountering and as possible solutions).

The first five years out of school really set the trajectory of your career. For the overwhelming majority of people, by ~30 they know what economic bracket they're going to live in for the long haul.

Sure, there are outliers (the median age of an entrepreneur is 42, the average age of someone launching a new private market investment firm is 51, etc.) where at the midpoint of adult life someone finds a way to move themselves from being a six-figure annual earner to an eight-figure or nine-figure net worth, but that's the exception.

By and large, your first two or three jobs prove to you whether you're going to make a lot of money or not. Illustratively:

• You do two years of banking at a top group in New York, two years of private equity, and nail a sick hedge fund role that lets you skip the MBA. You know you're more or less going to be pulling seven figures a year for the foreseeable future. If you become a PM at a giant shop (or open your own), hello eight or nine. It is now only a question of relative steepness, not whether there's a binary presence of steepness or not.
• You take an engineering role at Google, so you're earning high cash comp and gaining some valuable skills. 18 months in your roommate's best friend texts you asking for coffee and tells you he just raised a $3m seed round and is dying for engineering help. You're offered 1.5% equity. Six months after you start, you're a senior engineer managing a team of four who got hired after you. Three years in you feel like your weekend tinkering sessions have led you to stumble on an idea with a large addressable market, and since you've fully vested, you leave your role managing a team of 35 for your own startup. Your old company is worth $450m, your boss is worth over $100m, he knows you're rock-solid so he gives you $250k and makes some intros for you which wind up being a $2m angel round without any VCs. You don't know exactly what lies ahead but you and your co-founder own 40% each and things look promising. One year later you go to market for a Series A and get term sheets for $10m valuing your business at $80m. After the dilution and the option pool refresh you hold about 30%; you now have a paper net worth of $25m. One year later the prior startup you were at gets bought for $700m. Your original 1.5% was diluted down to 0.5%, but you just got a check for $3.5m cash.
• Your best job offer out of school is a financial analyst role at a 600-person company in the suburb of a secondary city. After two years you begin to interview for better positions, but because of the filtering mechanisms around white-collar opportunities, you're limited in how quickly you can upgrade at each successive step. You decide an MBA can help, but your success in the b-school cycle reflects the pedigree of your resume. Your options are Penn State, Vanderbilt, and Emory. You just got engaged, are planning kids in the 3-5 year range, and your fiance plans to keep working, so you can't take a banking role. The job offers you get are for $200k roles in various regional markets.

The point I'm trying to make is that even if people may not know the gritty details of each successive step that's available in front of them, at minimum they know that they're supposed to be nailing down what their adult life will look like.

You wanted to know "why people feel obligated to stop having as much fun" after school. That's it.

The other huge reason is inertia. It's incredibly easy to sit in a 9-6 job somewhere, come home and turn on the TV or Netflix or Xbox, fall asleep, and do it over and over again.

For some people that's a natural life. For others it sounds like death by boredom.

There's nothing wrong with wanting to do fun shit. I love doing fun shit and don't ever want to stop.

What I've found as my career has unfolded is that everyone's idea of fun differs, many people either don't have or don't want to spend money, life starts to be way more scheduled than spontaneous, family or social responsibilities stack up, and people are generally way less inspired creatures than in their youth.

My approach has been to make enough money that I can explore every single thing that interests me and transition to working the way and amount I want to.

Put simply, I wanted to have the money to do a lot of fun stuff (increasing the likelihood I'm able to do a thing someone else finds fun) and to be able to never have to worry about 'splitting the check', plus the free time to make use of it.

Turns out people are way more willing to do something if it doesn't cost them anything (no shit). If that means taking a NetJet with you somewhere, going out for the day on a dope boat, being front-row at the Drake concert ... you can see how you've now solved the 'why aren't people around to do things with me' problem.

If you follow this same path, let me caution you from experience that this doesn't eliminate your problem, it simply changes it. Before it was a sourcing issue (I want more people to do stuff with). Now it will be a filtering issue (I want the right people to do stuff with).

I'll be vulnerable here and acknowledge that I've outpaced a lot of my peers economically. Our culture is weird about talking about money. I wish that wasn't the case. The cool thing is that there's something of a natural sorting mechanism. If you pick the Vineyard as the place to buy a vacation house, chances are you have a lot in common with every other homeowner there.

So the answer to your second question ("how can I network to build some new friendships of like-minded people") is to begin investing your time in the things that you find fun. Then you're more likely to encounter people who won't be looking to take advantage of you; they're genuinely interested in the same thing you are and will contribute or reciprocate.

The tweak here is to pick scalable fun.

For example, brunch or 'Saturdays are for the boys' is great for a couple years. You're probably not going to be getting blackout on Saturdays when you're 35 though. You'll likely have a wife and maybe kids, and at minimum, your body's not going to take it and dealing with the hangover through the middle of the next work week will ixnay that for you.

If you like watersports though, that's something that will be with you forever. You can get your boating license easily and inexpensively. You can buy a jet ski. Maybe you can't afford a wake boat today, but you will be able to in a decade. And from engaging in that hobby in the interim, you'll naturally meet dozens of other watersports people who you'll be able to go out with, buy equipment from, go in on shared ownership with, travel the world with, etc.

Take that same approach to whatever it is you're jazzed about. Woodworking, gun sports, fishing, cars, dining, rugby, hiking, ultimate frisbee, whatever.

So to recap, the easiest path to an enduring solution is to make yourself the person everyone else wants to do stuff with. That requires you to be successful, so invest now in the professional success that helps you avoid finding yourself more or less locked into a comfortable-but-not-cushy life.

You don't have to be a zombie: "work hard, play hard" can be a cliche for the weird hardo everyone wants to avoid, but at its root it holds true.

You'll discover that the two are recursive: as you succeed in your professional life, you succeed in your personal life ... and the more you succeed in the personal life, the more professional opportunity lands in your lap.