As evil as they are, the answer to your question is that they simply don't have the logistic capability of launching a military invasion of Israel. These fucks are overrated, have dominion over a very small area and are in no position to launch strategic military operations hundreds of miles from where they are presently hunkered down. They are not a sovereign nation....they are are terrorist element that happens to be in possession of a "no man's land" area of Syria and Iraq. They should be exterminated as international outlaws. It would be wonderful if middle-eastern powers like Turkey and Saudi Arabia would take a proactive approach to these assholes and wipe them out instead of simply waiting for the USA to intervene like we always do. The new access that Turkey has granted and is participating in with airstrikes coming from Incirlik are a key move toward that end. Hopefully, this progression will be exacerbated and they can solve their own problems without the USA putting boots on the ground.
Sorry but this is just a moronic viewpoint.