Israel supports who is aligned with their regional objectives. Like when they supported Iran before the takeover. You work with what you got, there isn't big love between Israel and Egypt or Saudi Arabia and why single Israel? Everyone in the world has relationships with these 2 countries.
It's true that a strong Lebanon or Syria aren't in the best Interest of Israel military wise but at least there was someone in charge to keep the "peace" now it's chaos with Iranian backed groups working close to the border to destabilize it and Hezbollah has thousands of rockets pointing at Israel that's not in the best interest of Israel