Depression

You know you're not good enough to be a top academic at a good university
You're not that interested in money
The thought of being one of the mediocre masses with a normal office job and a normal life makes you want to kill yourself

What is the use of your pointless existence?

OP.... that sounds exactly like me. I finished a good degree at a good university, I was nearly bottom of the year, and I've done literally fuck all since then. I want to do a PhD but don't believe I'm smart enough.

We should make a suicide pact.

If you've done some kind of sciency degree that probably puts you ahead of most of the mediocre masses.