Can a 15 year old girl date a 20 year old guy in because ?

Dating only, yes, anything else, no.

Why would a 20 year old ADULT be interested in a 15 year old GIRL?

I think there's nothing wrong about that I mean its only 5 years apart not that much

There are no "dating" laws, but there are laws on having sex. Which is normally what 20 year old guys are interested in.

If she was 20 and he 25, no big deal, but now, that 5 years may as well be 30 years with the difference in interest, maturity and more.

Also without parents permission they may not either.