The driver might have realized (too late) what happened, if the driver noticed the second passenger left with a backpack they didn't come in with. Then gotten scared and decided to avoid calls and lie about things.

It shouldn't be possible to avoid police investigation just by avoiding calls. UBER should give all the details to the police and then the police should do their job and call the driver and the next passenger. Where I live (Poland), not responding timely to the police/court calls/mails would get you into big trouble and in the best case would end up in a fee and in the worst case you could be brought to the hearing by force.

That sounds like the sort of story a child would make up to explain why their hand wasn't really in the cookie jar.

Sure. I've heard of that problem, both with Uber and Lyft, on things that had nothing to do with fraud though. I don't know that either company does anything to reward drivers that answer calls/texts, or punish those that don't.