I think that this is the case (right now) because of Apple's stranglehold on the browser on iOS and the complex relationship between Google/Apple.
If FB could launch a browser on iOS that was in their walled garden, not only would it quickly receive wide adoption but it might become people's primary browser.
Not that I necessarily think that's a good thing, mind you.
Why would it quickly receive any adoption? Of all of the behemoths, I would trust FB the least here. Not that I trust any of the other big players enough not to use Firefox everywhere I can.
If FB could launch a browser on iOS that was in their walled garden, not only would it quickly receive wide adoption but it might become people's primary browser.
Not that I necessarily think that's a good thing, mind you.