This assumes they know and care that they're being tracked. Once faced with a clear choice without any dark patterns, it's fairly obvious that most users will select no tracking, unless the (supposed or real) benefits of being tracked are also explained to them. With the default Facebook behavior, they simply aren't being reminded of it. Out of sight, out of mind.

It's been known forever that opt-in data collection is almost useless, because the overwhelming majority of users won't allow it if they have the choice, regardless of the expertise. In fact, it's the reason for the current state of things - the telemetry and tracking of all sorts are being forced by default in most of the software and services with no option to disable it.

A lot of people don't realize they track you across websites etc, even if it might seem obvious to us.

I use the Facebook website from time to time, which I accept is kind of duplicitous, knowing the data they gather from those sessions will be abused. But my browser is configured to disable everything Facebook on sites other than Facebook (comments, "social plugins" etc) so at least it's confined and done explicitly by me, with my knowledge.

Because that's exactly what it says...

Consider "Would you like to improve FB experience by sharing how you use the app?"

You are a tech dude and read it the same, but it's not the same.