For reference, vscode ships as 13.3MB of uncompressed JS. (Not including builtin extensions, but that wouldn't add much)

CLA's are imposed by companies, not licenses. I'm quire sure that companies with OSS projects under GPL licenses would still want external contributors to certify that the code they contribute is theirs to contribute and can legally be licensed as GPL.

Indeed, there is the Developer Certificate of Origin for that:

https://developercertificate.org/

There's a very big difference between editing a file in a textbox and editing a file in an IDE pre-configured to have all the correct dependencies, extensions, build&test scripts, intellisense, etc.

Of course there is! But if it's literally one line...

The set of single line changes I'd be comfortable making with full IDE support is much larger than the set of one line changes I'd be comfortable making in a text box.