Why don't casinos allow this?

I've watched a few TV shows where Las Vegas security looks for cheaters. Can someone please explain to me why casino's do not allow someone (who is smart enough to do it) to win via card counting and doing other things that allow an advantage over the house. It seems like the house always has the advantage anyway. As long as a person doesn't bring something from the outside to help them, then why can't a person use their intellect to try to win? I've never understood this.