They can't unite for the same reasons that the North American Indians couldn't unite against the Whites...or the various tribes in ancient Mexico could not unite together against the Spaniards: Self-interest and old, long-established enmities divide them. Then too, they lack the effective backing (financial and military) of a Superpower, which Israel has. That always helps.

Also, Israel is only one nation.   Like a solo singer, it can't break up with itself, if you know what I mean... ;-) ...but bands which are composed of several individuals almost always end up breaking up, because the people can't agree on different stuff.

A single power, if it is strong enough to meet its enemies on equal terms (or in the case of Israel, better than equal terms), is a far more stable and effective entity than a temporarily patched-together alliance of several powers. It does not suffer from a divided command.