That was circumvented once by CDNs hosting common libraries so that those would stay in browser cache, browser vendors then "broke" that by caching per origin. (So that an evil site can't detect whether a user had been on some target site before by testing whether assets are fetched from cache)

Issue probably is that the standards process is slow (unless it is a feature Google "needs") and full of bike shedding (which features and how exactly they'd look) and adaption of features by developers is slow.

JavaScript meanwhile should be stable enough as an environment to allow a broader standard library.

Luckily it is slowly getting better (see Temporal as new date library, replacing moment.js usage in many places)

The web api is actually extensive. I can understand complaints about it being not exactly approachable, and some wanting a cleaner abstraction, but there’s no way that it is small. Most issues is about people wanting to download a small library than just vendoring the small snippet of code.

The other issue is the sheer amount of tooling and “plugins” for those toolings. Like the babel and webpack situation, which is truly kafkaesque.