You can easily understand why the TV should be low by going to the movie cinema and noticing two things:
most of the seats look down on the screen, not up
the few seats at the front that look up at the screen fill up last, they are the least-preferred seats.
People prefer to look down on a movie screen because it is more comfortable and less tiring physically for them to do so.
There are many articles that say proper ergonomic mounting of a TV is low. Here is one:
Both professional installers and ergonomic specialists have long discouraged mounting televisions above the fireplace because of the unnecessary stress it places upon the neck and shoulder muscles …
The Home Theater Mistake We Keep Seeing Over and Over Again
That article also shows the reason for mounting the TV high: because there is a fireplace or other fixture down low. Not ergonomics.
You also proved that looking down is more comfortable in your comment where you said you can half lie down on a sofa to look at a TV that is mounted up high. All you are doing there is finding a workaround that lets you look down on your TV even though it is mounted up high. You basically put the TV on the floor by laying down. That might work ergonomically in a bedroom, but in a room with couch and chairs you want the TV low.
Flip your question around: what ergonomic advantage is there to mounting the TV up high so you can bend your neck up to look at it? I can’t find any ergonomic study saying looking up for hours at a time is good for anyone. That is a position that we are advised to avoid no matter what the context.