Early attempts were pretty ham-fisted and only really useful for backoffice web apps with captive users and LAN connections. The past few years have seen a lot of advancement in tools like webpack to streamline how much code gets served for a given page.

And for a login page with what is effectively a form element with two input fields that IS is overkill.

Well, what was likely happening is that the entire application was bundled into a giant JS file that was served up on every page regardless of how much of it you needed.