[JumpCrisscross]

> how is this more of a filter than abiogenesis?

Common chemistries get us very close to molecular systems subject to evolutionary pressure. (Simplest: RNA world hypothesis.) We are missing links. But the pathway is plausible.

Chloroplasts, as you mention, are a potent counter argument. But once you have surplus cellular energy, additional endosymbiosis has a lower threshold. Based on current research, all life has a similar mitochondria. Different kingdoms didn’t nom their own and go. That uniqueness suggests difficulty.

[XorNot]

Or simply that success produces logarithmic returns: in the context of when this was happening, the first species to do it rapidly outcompeted all others and functionally ended exploration of the possibility space.