Yes, this exists today, but what Amazon would be buying is faster time to expansion. If they need drivers now (which is an assumption, I don't know this to be a fact) Lyft gives them an established network incredibly fast.

So obviously the price would have to be right and of course Amazon wouldn't be paying a large premium.

There are wild differences between flex and ride sharing.

Amazon already had an internal project that was going to compete with Lyft and Uber. It was called Rides and it was abandoned. Rex Tibbens headed that project and subsequently left Amazon and went to join Lyft as their COO.

yea it's trivial for Amazon to create their own 'driver network'