Because the originals parts where broken. I am speaking of 90s JavaScript, the DOM/Browser API and the not so much "batteries included". They could neither get fixed without a standard by committee approach of the browser vendors.

So we started to compensate: end state: Typescript to fix JS, React/Angular/Svelte to fix DOM API and a npm package hell to have batteries included. And all of that bundles, compressed, etc to have a good download time.