Why do smart people think they're so much better than normal people?

I don't think that's true.  Smart people are smart enough to know that being smart doesn't make you a better or worse.  Smarties  also realize what they do no know  and/or that there is much they do not know.   But then again, jerks and punks can possess any IQ on the spectrum. 

cause they like to think theyre better than anyone else

I find  illusory superiority occurs across the spectrum of mental prowess.

https://en.wikipedia.org/wiki/Illusory_superiority