essentially, cuz they don't know an alternative. and even them, if you told them about Steemit - the concept would probably seem so far out of their reality, they might not believe it or hesitate to jump in, doubting it could work for them.
nonetheless, here were are, and grow it shall...
In a way I guess that's a good thing. Limits the mindless clickbait and cat videos.
In the future facebook will just be an archived graveyard of badly cooked meals, cat videos and government propaganda. A reminder of how not to do it.