"best experience on Facebook right out of the box"
Except that never happens. The version of the app baked into the firmware is ALWAYS obsolete. So now your phone wastes space on a duff copy of the app and even more space replacing it with the current copy.
And if you never use Facebook you're stuck with it and whatever it's up to, e.g. stealing your contacts and usage info. At best you can disable this junk and pretend it doesn't exist. At worst you can't.
It's not just Facebook either but often other social media apps, antivirus software, dubious network/manufacturer services and so on. All cluttering the UI, draining the battery and reducing the chances of firmware updates.