But title bars are mandatory, so that's why not.
What does that even mean? That's the whole point of the discussion - standard UI elements have been bastardized so much, might as well just keep the trend going.
If there is a clean way to get rid of it, then sure that's a good option for some programs. (But there are a couple bits that you need to keep.)
But to ask "why even have it" is to fall into a discussion more about backwards compatibility than design.
No, it is a question about design. Window environments like Windows 11, macOS, Gnome or lots of other Linux window managers are designed for desktop PCs, not tablets. I would include Laptops under desktop PCs for this discussion, since most people -- that are working the whole day on their laptop -- add one or more screens to it. And on those large modern displays, it is more effective than ever to use overlapping non-fullscreened windows. Then it is absolutely necessary to have good standardized UI components to manage those windows. But those customized title bars break that standardization and customization. On modern Gnome I often have to search for the window that is in focus. It used to be clearly visible with a significant color difference in the titlebar. Today, there is slight difference in gray shading. That is ridiculous. And half of modern programs redefine the colors of their title bars, so you have to know for earch application which shade of gray stands for focus and which stands for not in focus. This is fucking stupid.
First and worst offender against this basic Windows guideline: Microsoft Office.
"Real" title bars can be removed if not needed (e.g. by switching to another windowing manager like dwm on Linux with X11; not sure how it's done in Wayland or in Windows but there must be a way).
"Crowded" title-bar from Gnome can't be easily removed, because they are not managed by the window manager.
They're mandatory because they're the way to move the window with your mouse. So fucking them up with a bunch of stuff that hinders that is just as bad as not having them in the first place.
Congratulations, you've circumvented the mandatory requirement!
Which is just as bad as disregarding it in the first place.
That's a screenshot across the entire width (of the top) of my monitor, as you can see from NP++ being maximised behind MS Word. Pretty darn busy, isn't it?
Sure, I can clean it up quite a bit, I can find out how to customise that. But naive users? They'll probably freak out. (And, TBF, I just noticed you can grab on to at least some of those busy-looking fields and drag it. But I only noticed that just now, while futzing around to screenshot it. And again, would my Mom? Or yours?) No, on the whole: What large software providers do to the Windows ittle bar nowadays is shit. The larger, the shittier. So of course Microsoft itself is worst of them all.