There's the pretty clear and obvious reasons of self-enrichment and looking out for national/regional interests in terms of geopolitical, strategic and economic advantages to be gained by having a friendly power in the region, but it seems like there's also a sunk cost they keep piling on in having to avoid confronting how wrong they've been.

Virulent pro-Zionism and intolerance of questioning this seems to me to come in large part from a western sense of guilt and shame over having allowed the Holocaust to happen while western powers sat on the sidelines for way too long, and they have collectively overcompensated on this front since the end of WWII, blindly doing anything to say "We're going to support the Jewish people going forward." Doubling down on this now means that they can continue to convince themselves that this was the correct, morally justified choice, while withdrawing this unwavering support would mean they would eventually have to confront the question, "At what point did the Israelis and the Zionist project become evil?" Unfortunately for them, seriously grappling with this question would ultimately require admitting that this isn't some recent development, but has been the case going back all the way to the very foundation of Israel, if not earlier. Without exceptional skills to manage this, admitting that the western powers have been materially aiding a genocide against one group of Semitic peoples to assuage their own guilt over having stood by to the point that the Nazis were able to advance an absolutely disgusting amount in their own revolting genocide against of the Jewish people (amongst many other groups in Europe), which almost no politician is willing to consider.

"So, to make up for how we enabled the Nazis and collectively screwed millions of Jews, we've been supporting a Zionist project that has been carrying out international propaganda and genocide for almost 80 years, to such great success in their propaganda efforts that Goebbels would be taking notes from it," is a hard pill to swallow. Instead of facing the facts, they double down on this and keep their heads in the sand when it comes to the horrors of the Zionist state and its aims.