This is a good example of making the mistake of conflating implementation details and user interface design. Firefox always had two boxes: URL bar and search. One of the big changes Chrome made when it was born was to have the Omnibar, being of course one bar that does both. If you think about this as an engineer, Firefox's design decision makes sense. You don't want to accidentally leak things to a search engine. UR…
Why don't they? This is a somewhat serious question.
Part of it, in my opinion, is we are moving too fast, and giving up on users to quickly. With enough education, people will understand. But enough education = at least a generation or two, introducing it in schools at a young age, etc. But we just threw our hands up and said "oh well, they don't get it, lets make it simple".
URL = phone number. Search box = Phone book. They are two very different things, and having them both in the same box I feel confuses some people even more.