Because they are zero rest-mass partciles
My question, and this was a question of some interest before I dropped out of my physics major and switched to philosophy, is what about the laws of the Universe make the speed of light the ultimate barrier. I mean, I can understand that something with mass would need infinite engergy to reach c, but why can't something without mass go faster? Do they have a tentative answer about that yet?
The more we answer the more we find we can ask.