Because they can’t generate enough self-worth by themselves. Therefore they must do so by demonising an outgroup, participating in the social action of scapegoating in order to create status within a group which privileges hate.
Infrahumanisation (or infrahumanization) is the tacitly held belief that one’s ingroup is more human than an outgroup, which is less human. […] According to Leyens and colleagues, infrahumanisation arises when people view their ingroup and outgroup as essentially different (different in essence) and accordingly reserve the “human essence” for the ingroup and deny it to the outgroup.
The idea that queer people are ‘less human’ is a rallying call to people who gain status from hate to care very much about the issue, because queer people become a potent target for personal and social disgust.