1. It's not enough when you're on an active connection doing other things at the same time as the video. Live streams at full quality will use a third to half of 20Mbps and demand very little interruption. Loading a single page with a few images can interfere. And even dedicated 20Mbps, with tightly encoded content, can be too little for 4k.

2. The idea in the comment is streaming at original bluray quality. Not transferring.

Maybe 720P, but I've got 4k Blu ray movies that have peaks of over 200Mb/s when streaming and my device always stutters over those sections.

20mb/s is not sufficient for 4k video unless it has been bitcrushed to death. And even then, you are assuming a given network only has a single user.

Also, I stream actual blu-ray rips over my network all the time, not just transferring them.

This feels exactly like "I don't have ethernet cards capable of 100Mbps so scp can't copy files faster than 10Mbps is not OpenSSH's problem".