Without rejecting any of the premises in your question I can come up with:

Low tractability: you assign almost all of the probability mass to one or both of "alignment will be easily solved" and "alignment is basically impossible"

Currently low tractability: If your timeline is closer to 100 years than 10, it is possible that the best use of resources for AI risk is "sit on them until the field developers further" in the same sense that someone in the 1990s wanting good facial recognition might have been best served by waiting for modern ML.

Refusing to prioritize highly uncertain causes in order to avoid the Winner's Curse outcome of your highest priority ending up as something with low true value and high noise

Flavours of utilitarianism that don't value the unborn and would not see it as an enormous tragedy if we failed to create trillions of happy post-Singularity people (depending on the details human extinction might not even be negative, so long as the deaths aren't painful)

Other reasons that people may have (I have some of these reasons, but not all):

• not a classical utilitarian
• don't believe those timelines
• too distant to feel an emotional tie to
• unclear what to do even if it is a priority
• very high discount rate for future humans
• belief that moral value is relative with cognitive ability (an extremely smart AI may be worth a few quitillion humans in a moral/experiential sense)

Of these, I think the one that I'm personally least moved by while acknowleging it as one of the better arguments against utilitarianism is the last. It's clear that there's SOME difference in moral weight for different experiences of different experiencers. Which means there's some dimension on which a utility monster is conceivable. If it's a dimension that AGI will excel on, we can maximize utility by giving it whatever it wants.

I'll add one more:

• Doomsday Argument as overwhelming evidence against futures with large number of minds

Also works against any other x-risk related effort and condones a carpe-diem sort of attitude on the civilizational level.

Here's Will MacAskill's answer.