I think israel is a useful tool in the hands of the US for a few reasons:

- A military local power that can be used to keep everyone around at bay. For Israelis this has nothing to do with the US but its more about "everyone wants to kill use so we have to fight/defend ourselves"

- A place to fuel the american military industrial complex. AIPAC and the general US public want the US to help israel against the bad guys. The US sends tax dollars (128mil in the last deal) to help out. Israel get US arms for the money they got. The military industrial complex is fed more money, saving jobs and keeping it alive. So overall its dax dollars --> military inducstrial complex. Everything else is used as an excuse, that is my opinion.

To make this whole story work, Israel has a specific image that needs to be kept. "We fight those that want to destroy us", which are usually the so called terrorist groups. Usually islamic, fundamentalist, crazy. They have to be a direct threat to israel for this to happen, if they don't threaten israel directly, then they are not interesting.

Look at ISIS (or Daesh how we call them). To my knowledge, they never stood up and said "our aim is to destroy israel". First reason for israel not to do anything.

Second (and more important), Israel (the Netanyahu government more specifically) can't afford going into another war. The general public saw the gaza story as a failure. Even though the media had been pounding the war drum pretty hard against ISIS lately, the resonance from the public was not the same as the one for the gaza story. So acting on it wouldn't be wise for Netanyahu, as he might lose more supporters.