"They cover common practices on the Internet, like showing an ad before a piece of content." Why do patents like that get granted? And once they are, why aren

"They cover common practices on the Internet, like showing an ad before a piece of content." Why do patents like that get granted? And once they are, why aren't they extremely easy to invalidate after the fact? There is a non-trivial aspect to a patent that is supposed to be passed. It seems to me that at least enforcing that would be a nice step.