Lowest common denominator. It doesn't matter how fast OUR connection is, it matters who has the slowest but pays for the service, because as mentioned, they need to support those people.

So we get average bitrates because: people. I think 4K streaming video quality isn't too bad, like on Prime, it is pretty good. SQ is OK, so if I want full quality audio, I buy the disc. When I hit about 30mbps, I get buffering over wifi.