I think the problem comes down to fingerprinting. For example, resources used to be cached in the browser regardless of site exactly as you described, but then ad companies figured out you can track a user's browsing history by timing the cache accesses to see if a user had loaded a bundle from another site. To remove that vector, browsers split up caches per domain.

That's a vector for literally all assets so adding a random delay to every layer up to the last Paint() is the only real counter.

It can be mitigated by adding a property to scripts to allow/default/deny shared domain resource caching, and the ecosystem can benefit from jquery/etc from being hot but still maintain opt-in default/backwards-compat status.

that, combined with the already established HTML <script> integrity Attribute and you are gravy

I believe one of the big reasons why things aren't cached cross-domain is the possibility of fingerprinting by looking at which requests are cached. It's also a lot less beneficial in today's (one could say unfortunate) world of JS and CSS bundling/compilation/whatever.