Because of their amazing bugs. I've bought the Sims 2 Pets on Friday and I was browsing a fansite forum, when I came across this post by a concerned player:
I had the weirdest thing happen after installing Pets. I created a dog and then called for an obedience trainer. They trainer sim came to the house but just sat in a van in front of the house sobbing and crying continuously! I couldn't get it to stop and I couldn't interact with the trainer sim. I ended up moving the family out and demolishing the lot. Has anyone else seen this?
That's such a great bug.
It seems that people don't have much luck even if the trainer is not upset, though, according to this answer:
My trainer won't train either. She came, I purchased training, then she sat on the couch and watched TV.