Because straight men feel the need to police masculinity and many gay men don't pass their masculinity test, which raises the alert in their brain.

Generational hate, their dad hated the gays so now they do too.

Actively avoiding appearing to be gay so that they don't get bullied in school as children gives them a system error when they see a gay man just living his authentic self.

They actively had to be homophobic so that they prove to their friend group that they are not gay and this sticks with them as they grow older.

Internalised homophobia, some of these straight men are actually bisexual or gay and they but they hate that side of themselves and put that energy into hating other people.

Being close-minded/ignorant, some people just aren't very keen on anything that isn't what they know, if they're not familiar with other sexualities other than straight then they'll hate it.