Simple: A great deal of the “content” it indexes – chiefly video, but also many newspapers and research citations – could be displayed only if certain rights or payments had been worked out. Owning its own browser, particularly one that works on all widely-used platforms, eases the engineering exercise of such rights/permissions/micropayment scheme.
Hence Google Firefox is quite possibly about rights and payments (that is, digital rights management) rather than commitment to open source or Web standards. (Google has no commitment to Web standards whatsoever.)
In the future, if you wanted
- to read an article from the New York Times or the ACM (which, like Elsevier and the other bastards, makes money by charging for research that was already paid for by someone else), or
- to watch video whose captions you had just searched
then you might have to use their browser.
For extra credit, ponder the question: Will you be able to watch that video with captions and audio descriptions?