I'd say because they're hard (or at least tricky and not trivial) to implement and rely on more infrastructure than a lean startup wants to maintain and once yo

I'd say because they're hard (or at least tricky and not trivial) to implement and rely on more infrastructure than a lean startup wants to maintain and once you do, you again have a separate service that doesn't interoperate with anybody, but now your technology stack is an order of magnitude larger than the competition's and therefore more prone to breakage.