You mean to ask why, SOME harem MC's are pervs?
Well because others aren't, also, I think it's a chinese archetype, not sure why, probably for comedic purposes.
Japanese MCs are generally shy, blushing constantly.
Chinese ones are pervs.
Korean ones sleep around alot, after doing lip service to being virgins.
In most chinese and some Japanese comedy ones, the giy is a perv, the girl ( even if a love interest ) constantly bonks him on the jead or refuses him strongly, and that's considered funny.
Not a big fan of it myself, I don't like it either, but apparently seeing a perv try and fail at being pervy is considered funny.
I guess it's funny. But only if it's done right. But more often than not, the overly pervy guy is cringe. For example, Juraiya from Naruto is a pervy goofball when he's not serious. He made me laugh a few times with his silliness.
Alternatively, I hate Minoru Mineta from My Hero Academia with a burning passion of a million suns. His entire personality is essentially "Pervy coward wants to be a hero for pervy reasons." No comedy + sexual harassment + pervy nature being their entire personality trait = Really annoying character.
Why are like all harem (maybe not all but) main characters utter scum?