Practically there's no difference between a browser that lazily downloads builtin fonts, and a global URL for a font file across websites with an extremely long TTL. And you don't want the browsers to be even more bloated than they already are!
Caches are no longer shared across domains because doing so leads to privacy leaks: https://www.jefftk.com/p/shared-cache-is-going-away
Enough so that if you rely on caching for user experience, your average user is not going to have a good time.
- not all sites use CDN, even if using the same font
- not all sites use the same CDN
- users clear the cache (clearing history does that on firefox)
- incognito mode is a thing
- first load matters a lot
- browser have a limit of the disk space they use for cache, and they evict older entries when they reach it. Given sites are now bloated, this fills up fast.
- this repeats for each browser. I have 5 on my laptop, 3 on my mobile, 2 on my tablet.