This is the "responsive" part of "responsive design". When done well, it's great, but you're right that many websites are way too aggressive about it, as if they only really tested two size classes.
I even had juniors maximizing terminals.
"We suffered for years waiting to get true multitasking and this newb doesn't even use it!" ;-)
The design developed at Xerox Parc.
Guess I must be a junior then.
Stop judging other people's personal preferences. It's their computer and whatever works for them is a good solution.
For me it is much better than multiple terminal windows...
Some websites are better maximized too, like maps for example.
I switched to a 52" TV ~2 years ago. It's the first time I found I didn't like maximized windows.
Personally, I find the way windows overlap distracting. It's labor to organize windows.