At the very least, you might need to edit the script that handles the POST request, in addition to the login form template. We're programmers so it sounds trivial to us. But it's already way too much for everyone else, because the average website owner will not spend any time trying to locate the POST request handler in the spaghetti PHP that his nephew wrote 5 years ago (and who knows what else will break if he edits that file).

I think a common login interface is important enough that we can't wait for the majority of websites to fix even a few of lines of code. (Does WordPress even allow plugins to modify the field names on the login page? It certainly allows them to add custom headers.) Copying and pasting a "Header add" one-liner in your .htaccess file requires 10x less programming skill and carries 10x less risk than rummaging around in a bowl of pasta, so it's much more likely to see widespread adoption. We could even open a service that crawls a given website and suggests reasonable values to put in their .htaccess files, for people who want the shiny new Login button to work in their websites.

I also don't see how it would be any more difficult for experienced programmers to update a header than it would be to update a template.