Because he's not sexually attracted to animals. He's sexually attracted to women, and gets something extra from the taboo nature of it. He likes human penor in human vagoo, specifically, but enjoys the degrading/taboo nature of incest. Make sense? You can't just check one box and get gratification from it, it requires a specific confluence of criteria to come together to create a "fetish" as a whole.
For instance, if your kink is degradation but you want hetero human-on-human, specifically, your biggest options are along the lines of bondage, anal/pegging, scat/waterplay, NTR/cuck, and incest. If you don't like bondage/assplay/cuck porn you're only really left with the one option.
A lot of the time people into incest porn aren't even into actual inbreeding, or attracted to their relatives, or anything of the sort, they're just looking for regular old vanilla dick-in-vag sex but with an extra taboo twist for excitement.