Generally because they try to be too clever, deciding what you want (or worse still enforcing it) because that is what most would want to happen. As far as I can see, Facebook postings and their visibility obey no normally accepted human logic. It appears to be solely based on AI deciding that, in order to ensure we are all subjected to the maximum degree of exploitation.
Even Mr. Trump and Mrs. May appear to have noticed recently! (surprisingly in both cases).
David