> Is there anything preventing this from being implemented?

Yes, normal browser security restrictions mean one site doesn't have the necessary access to another site. A browser extension can bypass these security restrictions (which also means you have to trust the extension's developer, since the extension has unrestricted access to all sites you open in your browser).

It used to be. Before the 4.0 release (which was shortly after Musk took over Twitter), the follow modal was much more user-friendly.

You'd get a popup telling you to enter your remote username (with autofill enabled), which then would redirect you to your actual instance to then handle the subsequent authentication and following process.

Gargron for some reason scrapped this modal when releasing 4.0s new UI (and in addition the suggested new method by copy-pasting URLs breaks remote following support in Misskey and Pleroma since those don't resolve raw WebFinger links out of safety concerns).

I would rather have feature allowing you to browse other instances from the UI in your home instance, so you don't need to go off-site for that.