Yep, for the same reason you won't find a lab guarding a junkyard, pomeranian working as a bird dog etc... etc...
It is a nature thing, and most breeds were developed with certain characteristics in mind.
Add to that now, that frequently people are breeding for looks within a breed (especially bully breeds now), and you exacerbate the problem because you can throw confirmation and temperament out the window.
We get a lot of calls even out here in the country to do cesarean sections on bully breeds that are now incapable of naturally delivering puppies. Our clinic policy (my wife and I) are that we will do the C-section, but we're also doing a spay at the same time because you're promoting unhealthy and unnatural animals. It pisses some people off who go elsewhere, but we don't really want their business anyway. Think of it as our passive-aggressive way to weed out undesirable clients.
The owners of theses breeds are frequently just as stupid and unpredictable as their pets, and it just isn't enjoyable having them in the office in the first place. 95% of them just see their puppies as a chance at a paycheck anyway. Literally the bulk of their conversation is centered around how much they can sell each puppy for. I wish it wasn't like that, but it is.