Invalidating patents with prior art isn't as easy as you might think. There are two ways to do this: you can invalidate on novelty grounds (that is, the invention was anticipated by a single piece of prior art) or non-obviousness grounds (a combination of prior art essentially undermines the idea that the patentee took an 'inventive step'). Only the claims in the patent are considered when assessing novelty and non-obviousness. In the case of novelty, a single piece of prior art must embody everything claimed in the patent, whereas non-obviousness loosens this restriction. Non-obviousness claims are typically far more difficult to prevail upon, which is why companies won't do it more often.
For what it's worth, the relatively recent KSR v. Teleflex case gives us the current opinion of the Supreme Court in non-obviousness battles. In that case, they slid the scale back to favor the Graham[1] test for non-obviousness, rather than the controversial teaching-suggesting-motivation test, which tries to determine whether something in the prior art suggests the desirability of combining the claimed subject matter, and thus its obviousness. Now, the Graham test (which was a 1966 decision) allows for empirical evidence of non-obviousness, to wit: commercial success, long-felt but unsolved needs, and failure of others to invent the patented subject matter.
All of these are issues that need to be litigated in federal court, an exceptionally expensive endeavor that no corporation will take without a very strong incentive.
For what it's worth, the relatively recent KSR v. Teleflex case gives us the current opinion of the Supreme Court in non-obviousness battles. In that case, they slid the scale back to favor the Graham[1] test for non-obviousness, rather than the controversial teaching-suggesting-motivation test, which tries to determine whether something in the prior art suggests the desirability of combining the claimed subject matter, and thus its obviousness. Now, the Graham test (which was a 1966 decision) allows for empirical evidence of non-obviousness, to wit: commercial success, long-felt but unsolved needs, and failure of others to invent the patented subject matter.
All of these are issues that need to be litigated in federal court, an exceptionally expensive endeavor that no corporation will take without a very strong incentive.
[1]: http://en.wikipedia.org/wiki/Graham_et_al._v._John_Deere_Co.....