The industry couldn't decide what it wanted for a long time (and maybe still hasn't settled). Pages needed to be consistent visually no matter which platform and browser the page was displayed on (which is odd, few people used the same sites on multiple platforms at the time). Oh, but then it was visually jarring because it didn't match the rest of the platform and you couldn't leverage your knowledge and experience of the platform's UI (the way multi-select worked in drop downs, accessibility). But then you couldn't style the widgets on the page to make your site distinct (styling OSX "lickable candy" widgets didn't work so well since they were so visually unique). And then you had mobile where space is a premium and scrollbars were sometimes visually hidden and the entire application took up the whole screen, so differences from other sites and the platforms as a whole were not as prominent. So all widgets had to be stylable (IIRC, you could style buttons but not scrollbars for a while, which made things inconsistent within a given page itself). And complete control of the "experience" became desirable, the scrollbar was seen as part of the page, not part of the container that holds and displays the page.

Ultimately the browser was recognized as its own platform and had to support all the customizations and accessibility concerns. User stylesheets are a thing, but few used that for much more than hiding annoying elements, and today the most likely user customizable thing is being able to switch between light and dark modes.

I'm not sure you're actually describing user's expectations, I think you're describing an oft-held belief about user's expectations among people who fancy themselves UX designers.