I feel your rant, I really do.
You have no idea how disappointed I was after the Wikipedia redesign until I found the full width button in the bottom corner.
Most sites are optimized for mobile and are completely asinine looking on a monitor.
Especially text heavy sites where even a single sentence is broken into 2 or more lines, meanwhile 70% of the screen is empty.
And it's not like it's hard to implement a button like Wikipedia did, web designers just don't give a crap.
I payed for a full monitor, let me use the full monitor!