Because a lot of them aren't. A lot of it is the physics, ragdoll/natural motion stuff.
Either you have a game full of canned animations with less control (funnily enough the two games mentioned in this thread 2K and FIFA are both guilty of that) or you have a looser animation system like Live has been building over the last few years.
Visually, it doesn't look as good as it could when going down this path but gameplay wise, I'd take it any day of the week. The precision and responsiveness is second to none and the unscripted animations made possible by their physics system (having played 14 and 15 for about a year each) really give the game a look and feel no other basketball game has even attempted, let alone matched.
And while I agree some animations most definitely look 'off' (the dunks and shots around the hoop especially, simply don't resemble NBA athletes), 'stiff' is far from the word you use to describe Live. It's almost too loose and the opposite of stiff and robotic.