This is, I understand why we're in this mess: Nobody really has the time or resources to run enough controlled tests to obtain enough quantifiable data to get a definitive answer. But that's fine, it's a hobby, not a job, after all.
I think you've hit the nail on the head. People do what they do because it's what they were taught by someone or advised on places like this.
For most people, fermentation temperatures are what they are. You put your beer in the basement and it could range from 65-70 depending on whether it's winter or summer. If you want to ferment at 63, that's kinda too bad.