I’m trying to understand why they(we) are such a close ally

You won’t like Biden’s answer…

Biden has partly credited his pro-Israel world view to his father, who insisted following World War Two and the Nazi Holocaust there was no doubt of the justness of establishing Israel as a Jewish homeland in 1948.

https://www.reuters.com/world/us/i-am-zionist-how-joe-bidens-lifelong-bond-with-israel-shapes-war-policy-2023-10-21/

He’s said multiple times there is no line Israel could cross that would result in any less support from him.

He’s not operating on any kind of logic, in his eyes Israel = good, and that’s as deep as he will ever think of it.