1. They bloated the app so badly, they would have had to monkey patch the OS to let it run at all.
2. They actually did have the stonkingly huge arrogance to monkey patch the OS.
What they should have done is shattered it down into a constellation of related apps and background processes. This app is FB news, that app is FB messaging, and so on. Bonus: people could pick and choose the parts of the system they cared for.
But that last is what they wanted to avoid, I think. Facebook à la carte means that they can't foist unwanted "services" on you.
And so they hacked.