Facebook/YouTube don't care about how their compressed video looks. They just need it in the format so that they can get in front of their millions of user's eyballs. I'd be willing to bet that >95% of users don't "care" about compression quality. They just want the content, hence their decisions. There's just no way to properly encode that much content with a "we care about compression" thoughts.
Why is that? Isn't 15 mbit to 2 mbit a big enough difference in quality that you'd expect to retain all the good parts?
Not really - "raw" 1080p video is ~3gbit/sec - so going from 3000 to 15 is a huge amount of data compression.
The problem is when you go from 15 to 2 in my example is that the encoder spends all the time trying to basically encode the artefacts. It really doesn't work well at all.
The quality from going from source -> 15mbit/sec h265 -> 2mbit/sec h264 (like on a classic social media or whatever site) is absolutely terrible compared to going from source -> 2mbit/sec h264.