Why aren't browsers also editors?

Answer 1

Netscape is both.
So is seamonkey. available from the folks at firefox.

Answer 2

Some browsers have both, but these browsers are very large downloads and take a lot of room. And for simple HTML editing, you can use notepad or other simple text editors. If you mean what you see is what you get (WYSISYG), those editors are more complex and take up a lot of space, like I said earlier. Most people don't want the editor, so not including it makes the browser smaller.