I never got the hate for Boostrap or other frameworks. They definitely save a lot of time, and you don't necessarily end up looking like every other site if you put in some effort[1].
Properly theming (at least in 3.x) took a lot of effort. Just try to make a theme whereby at first glance, you can't tell that Bootstrap was used. It's not that it's not possible to do, it just took a lot of time and effort to nitpick over every detail.
In professional environments (ie: real flesh and blood companies), developers cannot say "we're using Bootstrap's default theme". The business has designers, the designers come up with a design, and unless the developer can customize Bootstrap to accommodate the requirements, in a reasonable amount of time to meet deadlines, Bootstrap is simply not an option outside of using the grid system.
That's precisely what we do at my place. We don't do it for customer facing things, but for back office tools why would we spend time customising Bootstrap? There are better things we could be doing with our time.
How terrible would it be if all your desktop apps had completely different UI components? Indeed, people seem to greatly prefer desktop apps that follow the style and UI conventions of their OS.
So if the Internet were largely made of pages powered by a few popular UI frameworks, I really think it would be a huge boon to users everywhere. UIs would be more familiar and consistent. Seems like a great step forward.