[retrocryptid]

Plenty of reasons:

A. There are several types of experiments that could be easier outside of emacs than inside emacs.

B. If you wanted to extend the browser with Javascript instead of e-lisp, rewriting it as a node package or electron app might be easier.

C. To scratch an intellectual itch. How do you know what's possible if you don't try?

To be sure, it would likely take a LOT of time before it reaches feature parity or attracts as large a community as emacs, but it sounds like a fun project.