Do I need to network to get an internship/job in the following fields?

Yes, of course you need to network...alot

I know people who get offers without networking - but they go to target schools and have excellent GPA (as in 3.8+)

Networking can only get you so much. You can't just rely on other people to get you a job - most of the success still comes from you (e.g. past internships, good grades, well structured resume...etc).

Contrary to popular belief, there is still meritocracy left on Wall Street. I never bought the argument that I had to rely on other people to get me in the door.

It's not only about networking, it also has to do with luck ( whoever read Monkey Business understands what I am saying). But yes, you need to network, every time you speak to a new person you will learn something new, although they might not secure you a job position.