Because a dog has been a part of the human social hierarchy in many cultures. Calling someone a dog alludes to various social characteristics. Calling somebody a cat is just non-sensical.

I suppose there could have been cultures where a cat occupied a place in the day-to-day social hierarchy (and not just in a religious sense). But if so that sense has been lost to us. For obvious reasons I wouldn't expect such a relationship to have been anywhere near as pervasive as with dogs.