Some people do! Check out the top story on HN right now, which is about EFF invalidating a patent held by a patent troll: https://news.ycombinator.com/item?id=9356767. Nobody was suing EFF (admittedly, that's almost a technicality since the patent holder was suing others, but EFF was trying to advance the public interest).
Still, it means that, if Facebook sue you with stupidly-obvious-invalid patent on entirely unrelated product, say a patent that car must have a glass windows, then you can't even defend that such patent is invalid.
Also check out Ask Patents, which works on invalidating patent applications: http://www.joelonsoftware.com/items/2013/07/22.html.