I had a "hide sidebar" bookmarklet for Reddit that lived in my favorites bar and got used dozens of times a day. I didn't mind so much when Reddit was fullscreen, but when I was cramming it in with other windows on my desktop that extra real estate made a huge difference.

Personally I see the value of the sidebar but I think it would work better as a fly-out UI element that can be reduced or hidden, and stays that way until opened up again. I'm in the process of trying to stand up my own instance to see how that could be done.

I general why does there have to be static sidebars that are rarely used. It causes the content body to be squeezed into tiny space.

I think the rationale is that most people use widescreen monitors nowadays, so if you allow the content part to run across the entire width of the screen, it becomes ugly and hard to read. Therefore the middle section gets a limited or fixed width, which in turn then creates two empty columns to the sides that designers are then tempted to fill up with "useful" stuff.

You can try this yourself: paste a long line of text into a notepad window and maximize the window. It is much harder on your eyes to read and focus on the text than if you resized the window to a more reasonable width where the text gets broken up into several lines.

I'm not against this design paradigm per se, but the content width reduction is often overdone, leading to a squeezed feeling like you say. It can also create problems if you have a habit of not using maximized browser windows, but for example a window tiled to one half of the screen. Some of the better sites work around this by having a reactive design that reduces, collapses or removes the sidebars when the window is narrower than a certain width, but many sites don't.