I suspect if you were willing to completely go to war over a hire, you could ultimately get noncompetes invalidated. The problem is the chilling effect on marginal hires -- it usually isn't worth going all the way on hiring someone when you have other options, and even when it is, it makes sense to settle and move on (for both sides).
(From what I've heard, Amazon is particularly egregious in enforcing noncompetes. I believe they are an unconscionable and unconstitutional restraint on individual liberty, and believe the federal courts would agree, as well as the court of public opinion.)