It’s the combination of big imaginations and little real-world experience. In Friendship is Optimal, the AGI goes from asking for more CPUs to asking for information on how to manufacture its own CPUs, somehow without involving the acquisition of silicon crystals or ASML hardware along the way. Rationalist writers imagine that AGI will somehow provide its own bounty of input resources, rather than participating in the existing resource economy.
In reality, none of our robots have demonstrated the sheer instrumentality required to even approach this sort of doomsday scenario. I think rationalists have a bit of the capitalist blind spot here, imagining that everything and everybody (and everypony!) is a resource.