Considering how old Facebook is…. They probably never bothered to upgrade the authentication system because “if it ain’t broke, don’t fix it” and it didn’t matter to their revenue.

Facebook is huge and has very diverse teams/departments. It’s absolutely possible the guys who know what security is, and the guys who build app xyz are in different departments, countries, continents.

The capitalists want us to believe otherwise, but large corporations are just as convoluted and inefficient as a planned economy.