They get it done because they want to. Because they see something in the mirror they aren't happy with and want to change. Or because they make a mod they think might get them more scenes. Or on occasion, they make changes to please others (I have conversed with one pornstar who got bigger implants because she knew her then-husband like huge tits). It's not rocket science.
I think you knew these answers already and just fancied a moan. It's no big deal. Once a performer makes a change that you don't like, then Foo Fighters her ass and be "done, done, on to the next one." No point whining about something that can't be changed. Plenty of smoking hot all-natural girls enter the biz every day.