I think its one of those strange language things, not necessarily laziness. If you say would've or should've outloud they really sound like would of and should of, and since language is taught verbally much more than written, its just been "adapted" over time.
I am sorry, but I disagree with this. Yes, it is how it sounds when said verbally, but we should still be teaching the correct written grammer to our children (or peers who are learning English, or how to read/write etc). I don't think it has been adapted as a correct form of grammer.