As much as I'd like retainers to be able to very slowly bind gear, I'm pretty sure the only reason they bind to 1% is so that you can't resell it as you've "used" it, its same reason that gear hits 1% if you use it for a glamour.

I'm also sure there is the possibility that retainers aren't actually "people" and are in fact a type of mimic that has integrated itself into society with a more complex disguise and less tendency to eat people why else would they want so many venture coins and look and act however you want! Its a conspiracy I tells you!