Because people think their app will immediately scale to a billion users and their database is unable to service all those users. (It won't and the database can

Because people think their app will immediately scale to a billion users and their database is unable to service all those users. (It won't and the database can) Alternatively, they have drank the microservice kool-aid and think that making a request to an AAA service every time the user wants to do something is just too much overhead (It isn't).