How in the world does she have 23 million followers? When was she last relevant?
For the same economic reason you can get forty telemarketer calls a day: the cost of "following" her (or phoning you) is effectively zero.
Some statistics that would be interesting, if one happens to like stats:
Percentage of Britney's Instagram followers that are active accounts for which the user generates personal content.
Percentage of total Instagram accounts that follow Britney.
Percentage of total that are females between 15 and 45.