Partly, the answer is “tough”. As a designer, you don’t and aren’t meant to have pixel-level control over the screen contents. Web is not print. Don’t ask for the PostScript standard fourteen. (Somehow this lesson comes through much better for reflowable ebooks.)
Partly, I am willing to admit that web fonts are still nice when you can get them. But they’re too unwieldy to block on (slow connections exist; font foundries are assholes[1]; etc.), and we don’t really have a solution (the problem with FOUC is not the unstyled content, it’s the layout shift).
While I'm absolutely not a design-should-rule-all person, I think there's quite a range between "pixel-level control" and "you can't choose which font to use".
If we'd reliably have the top 50 google fonts on every OS, there'd be a lot less webfonts used.
system-ui
Glyphs are taken from the default user interface font
on a given platform. Because typographic traditions vary
widely across the world, this generic is provided for
typefaces that don't map cleanly into the other generics.
Until OS developers get their house in order, designers will keep on ignoring them.