They do strike. I'd imagine the reason you don't hear about it as much is because when buses strike, chances are you can find your way into work using an alternative without too much trouble. Whereas with tube and train strikes, alternative transport is always more hassle, therefore generates more press interest. This is true for London, anyway. Also, RMT and ASLEF are v. powerful unions with good funding, therefore their press office & spokespersons are probably more media savvy.