Most users do not want to configure fonts or anything really to use the web.
There are certainly people who like to tweak things a little, and then there are the tinkerers who want to use custom fonts or other changes. I wonder if the customization slowed down as computers for work got more standardized? Or maybe interchangeable is a better word.
The weird thing is, whenever I see a custom font (or at least when it’s noticeable) on a phone, it’s Comic Sans. It’s usually people who are not super tech savy, but still they went out of their way to customize their phone to show them all interface elements in Comic Sans.
As the expectation became "We should design our products so that the most technologically inept person can use it" customization in general got sidelined and a huge number of users are either unaware of what customization options still exist or lack the confidence or capability to take advantage of it. The result is that for most people's stuff, everything looks standardized and a small number of folks (the ones who have always loved customizing and personalizing everything) are the exception.
Perhaps those of us who like to customize were always doomed to be the exception rather than the rule since sticking with defaults is the path of least resistance and as long as something mostly works many users won't bother to change anything at all. It's also harder for the pro-customization crowd because eventually developers are going to consider something along the lines of: "The majority of our users use whatever defaults we impose on them, and having things more standardized can make it easier for support anyway, so why are we spending time/resources maintaining all these customization options that only a few people use and which could potentially make a clueless user confused and angry." which can lead to further restricting/limiting customization options.
It can get much worse. I have a friend who uses a "cute", handwritten-looking font on her phone. She is Chinese. When she sends me screenshots, I can barely read them. I have enough trouble with standard characters.
QQ took that one step further, and let you customize the font in which your messages appeared to other people. I don't think QQ is as popular as it was, though.
But they're complaining about me, not their browser.
A simple browser-provided theme dropdown containing a few nice options, directly in the main browser chrome, from a major browser (perhaps Firefox, back when it still counted as "major") could have changed things completely—see the pressure on sites to support "dark mode" now that that's a simple option to enable.
It'd be nearly impossible to change now, but 10-15 years ago, maybe it could have been changed, if any of the very small number of entities steering the direction of the web had tried.
It was a UX problem, not a problem with the core concept.
Now, decades in to The Web, it's also a chicken/egg problem, because almost no sites are designed to behave OK under reasonable customization by the user, and almost no users customize their browsers' default styles, so why would sites change to accommodate that? Aside from the light/dark thing, of course, and you do see users pushing for support for that, and sites putting in effort to support it.
If Apple expands Dark Mode to include a half-dozen other options for a11y and such, I bet you'd see support for those become fairly common. It's just got to have a decent UI and the push has to come from a browser with a large enough user-base to encourage site operators to care. Once upon a time, Firefox could likely have done it, assuming they could manage not to screw it up. These days, Apple, Google, and MS are the only ones who could realistically try.
Actually, we have another version of this, now that I think about it: reader mode. People seem to really like it.