It's about control. On my smartphone, its easy to start using google plus instead of Facebook. If I had bought a Facebook phone, they could lock it in to Facebook services so that i use Facebook as my primary social network for at least the life of my phone.
Also, it's a revenue stream. Facebook has a huge userbase but very few of their users actually pay them money. If they started selling hardware they could collect money from those users.
Also it's about the control that Facebook, like Apple, might be able to deliver better UX if they could design for and thoroughly test on very specific combinations of hardware, instead of trying to function across so many different chips and screen sizes.
But Facebook, unlike Apple, has no practical choice about operating across those many different chips and screen sizes. Social networks are driven by exceptionally strong network effects. If Facebook doesn't reasonably operate across those chips and screen sizes, they run a serious risk of losing out to a competitor that does, especially since there's a significant one waiting in the wings.
Facebook needs to fire their entire mobile app development staff. The mobile app does everything the desktop app does minus Push Notifications and yet takes easily 10-20 times as long to load. It's pathetic. There is no good tablet app, etc, the list goes on and on.
The idea that they're going to make their own OS and apps that are as polished or feature complete as their competition which is already possibly too statured is hilarious at best, sadly delusional at worst.
Also, it's a revenue stream. Facebook has a huge userbase but very few of their users actually pay them money. If they started selling hardware they could collect money from those users.