I remember bikeshedding with my coworkers about this topic a few years back while coming up with lint settings. We ended up analyzing all current line widths and realized that something like 98% of lines were under 110 characters, so that is what we ended up using.
I think a lot of terminals just set that as the default window width. No real reason for it, I usually snap to half the screen width immediately anyways. That turns out to about 105-120 characters width. Some people might just not resize it most of the time. Especially on Mac where efficient window management just isn't a thing.
A note on your last sentence. Once I used my Win10 laptop at an interview at Apple. The interviewer saw me drag one window the the side, snap it, and then select another window to fill in the space.
He said, wow, Windows is really nice! :)
This should be standard in Windows IMO. It has absolutely transformed how I work and noticeably increased my productivity.