Probably because it's same group of people that defined standards like media queries and wrote "best practices" ...

Technically browser would need to report different pixel density or resolution (check what https://www.whatismyscreenresolution.com gives you) to get "desktop looking" site.

Though in reality it's one/same site - so I'm not sure how much would you get with that...

The worst is when some consultant designer does some journey mapping exercise, dropping half the functionality of the site and disappearing.

My bank recently did this, they decided that “complex” workflows like seeing how much interest you paid or transferring money wasn’t in their top 10 transactions that were journey mapped. The consultant probably took the fancy post it notes with him.