All these people hate Facebook, but they still use Facebook. How many of those one-star reviews were written by people who still launch the app 20 times a day? What incentive does Facebook really have to change their QA practices?
But the flipside of that is that if it becomes too bad, then the first people who start leaving weakens the network effects and cause more people to leave, leading to a chain reaction and a swift collapse.
Facebook's incentive is to prevent such a collapse.
It runs me off the mobile app, I use the web interface.
Also the huge list of Permissions it requires.
I'd say it's a mix of people who don't equate the Facebook app with the Facebook content, who don't like Facebook as a company, but loves the content and who don't feel that their recommendation is necessary for spreading the app (that is, one can give it 1 star to punish FB for my app crashing, but I still recommend friends to download it)
Think of it like a politician might: once you've donated to his campaign he has no incentive to give a fig what you think.
Of course people with lots of clout and wealth are special cases and the above don't necessarily apply to them.