What customer base (at least in the US) does Lyft have that Google or Amazon don't? Estimates for Amazon are ~250M and Google ~275M users in the US.

it's not only about number of users. You also have regulations in different cities and states, existing models trained based on past supply demand behavior, somewhat optimized workflows for the ride hailing industry, payments, risks.

They both can build themselves, but if you provide solely self-driving ride hailing, a lot of times customers might not be able to find cars in upcoming 6-7 years until they ramp up full production to meet demand.