Discretionary Prop Firms Frowned Upon

Why is discretionary trading frowned upon? 

I understand it's too risky for many, but isn't the upside potential and freedom greater than almost any other job in a trading related field? 

I've had the opportunity to speak with a few traders and got mixed answers about their experience.

The ones that have stayed long enough were happy with where they are and said it's totally worth it. 

Then you've got the guys that are a few years in and can't get anywhere despite their high educational background, and looking to get a safer job with an actual salary. 

And last are the guys that are just starting, highly optimistic, maybe still in their parents basement and possibly with expectations that are set too high, yet time will tell whether or not they can succeed.