I don’t think it’s a baseless accusation to suggest the US stands to gain something from the relationship, and suggesting otherwise is a little strange considering just how robust that support is.
Iron law of institutions. Institutions don't do what benefit the institution's hold on power; institutions do what benefits the decision-makers' hold on power. The US supports Israel because Israel can swing elections, especially Republican primaries and close elections, in favor of pro-Israel forces. For that reason, most politicians have become pro-Israel. It also creates a feedback loop - politicians feed pro-Israel sentiment because they are (now) on the record as pro-Israel, and need as many voters as possible to be onboard with that.
I’m assuming that didn’t come out of nowhere.
It didn't. It came out of Israeli funding combined with evangelical ascendance into politics in the 80s. We were ambiguous at best in the 40s, 50s, and 60s, and only mildly supportive of Israel in the 70s.