Easier regulatory environments. Not only do you not need to talk to the FDA before testing your drug in animals, cats are way less likely to sue you if they get sick from your drug. Of course, we’d still do our best to make sure they didn’t, but it’s less likely to sink the entire company if they do.
And that's how our dog died in 25 days from a painkiller that the protocol is to check for liver function after 30 days... I suppose this sentence is meant to be a little tongue in cheek, but a few of us at least do not find it funny.
Still: our society (rationally) does not devote the same resources to drug development for animals as it does for humans. And even protocols, dosing regimens, etc, for humans are not perfect. And even when the ideal protocol is chosen, some people still have a bad outcome by being an outlier.
Medicine is very imperfect; veterinary medicine is worse.