probably because pigs are not exactly graizing animals that can live off of ~grass like horses/cows/etc. Pigs would in this game have to be fed or made to 'gather' stuff like mushrooms and roots

i imagine colonial charter or something like that could have more animals

Colonial Charter has pigs indeed (and tons of other things and stuff and beasts and omg too many things to list). We just imagine that people feed them whatever food wastes they have from the previoust day's meals on top of whatever the pigs can find in the pastures. ;)

I don't remember seeing a standalone pig mod though.