> preferring Earth to these small bodies seems to me to be mere prejudice.

I really don't think it is. For any favorable condition you might find in asteroids, early planets can probably match it, and a bunch of other possible conditions besides, with the additional benefit of not requiring a stage where the nascent life has to survive an impact at orbital speeds, subsequent flight through cold irradiated vacuum, and then re-entry through a dense planetary atmosphere. I'm not saying it's impossible, but I'm saying it needs a lot more evidence to take it seriously.