You can hide it for the same reason people choose to hide their bookmark bar, or reading list, or dock. They simply don't want it to take up room on the screen so they can focus on the website
It‘s okay to make it hidable as a user preference setting, but the default should be for the scrollbar to be visible (and wide enough to not be a difficult touch/mouse target), for affordance and discoverability reasons.
What if Apple took your argument to the extreme and remove the URL bar, back button and the tabs bar so you can focus on the website? Would you be OK with that?
A) they haven’t removed the scroll bar, they simply hide it when you’re not scrolling.
B) Despite your cheery tone that IS an option you can turn on when you fullscreen safari and Chrome, and I do in fact use it. It’s great. The whole screen is just the web page you’re on.
You mean those wide-load, content dense websites like this? https://imgur.com/a/6pQAXWP
BTW the only site I know what supports wide-load toggle is https://tvtropes.org/pmwiki/pmwiki.php/UsefulNotes/UNIX
And F11 still works.