Why is there cow cheese but no pig cheese?
There are several main reasons, historically:


Cows, sheep and goats have udders, and can be milked by hand into a bucket. Pigs (or cats, or humans for that matter) have nipples, where you need to suck the milk out. Consider that most of the cheese in the world was invented before machinery was able to create a vacuum, this means that the only option would have been to suck milk from the pig and spit it into the bucket.
For making cheese, the fat/protein ratio in the milk is important. It changes, obviously, from species to species, which is why there aren't many hard yellow cheeses made of goat's milk.


Lastly, I have actually had camel's milk ice-cream, which goes to show that it's just a cultural thing mostly.