How and why are pets infertile?

Pets are made infertile using potions of infertility or the exceptional infertility kit (both items sold in the diamond shop).

Why pets are made infertile: usually to protect the hard work and money/time investment put into breeding projects. For individual non-stat projects, this is usually to stop people from purchasing a completed project offspring and breeding to a similar (or dyed) unrelated pet and underselling/stealing the design of the original breeder. For stat projects, this is usually done to protect the entire stat-breeding economy. Max stat pets take a lot of time (think more than a year) and a lot of monetary investment (for philters, stat boosts, gender swappers, stable space, etc) to complete. So, if someone could just buy two fertile max stat pets and breed them, then sell them for far, far less than other stat breeders are, the entire stat breeding economy for that particular species could collapse.