because he's not smart, his belief that he's of a superior intelligence is a last desperate grasp at having something to feel good about. I mean look at him he's a fat junkie who lives in his mom's sewing room with no education no job experience no marketable skills and no love life to speak of. he's never, ever, going to be the best at anything, he'd have a tough time even being adequate. so he spins this tale to himself about how he's a misunderstood genius and we all just think our lives are better than his but in reality he's unlocked the secrets of the universe and is the only human being capable of seeing through the bullshit to the mathmatical truth of reality.

stevey has a very similar form of neckbearditis but instead of "I read a mathbook I'm basically stephen hawking" stevey is more like "I could be a millionaire with like no effort but it's stupid and beneath me and living in a van is way better because reasons"