But they do. Javascript itself has evolved significantly over the past decade obsoleting many features that used to solely be provided by add-on frameworks.
HTML5 as a living standard brings the majority of the needs that used to be served by plugins – interactivity, dynamic pages, two-way communications, multimedia.
WebSockets provide for realtime communications with the browser, something again not possible without 3rd-party plugins.
So, what you are saying is in fact happening. But that doesn't mean it's not an issue with sliding goalposts. It's just that it doesn't happen as quickly as we all may like it, but that's because "basic functionality" is a constantly moving target with different definitions depending on who asks.
I cringe when I think about how many petabytes of jQuery has been sent over the wire over the years.