Perhaps because they find it too inconvenient:

If Israel sent the IDF three kilometers into Lebanon and started digging trenches and building bunkers it would make news all over the world. But Syria does it and everyone shrugs. Hardly anyone even knows it happened at all.

Syria can, apparently, get away with just about anything. I could hardly blame Assad at this point if he believes, after such an astonishing non-response, that he can reconquer Beirut. So far he can kill and terrorize and invade and destroy with impunity, at least up to a point. What is that point? Has anyone in the U.S., Israel, the Arab League, the European Union, or the United Nations even considered the question?