Its just that her fanbase are really strong and loyal. Young white girls who can relate to her and see her as a best friend. They don't mind streaming her stuff 24/7 on multiple platforms and buy each and every version of her records.
She however will never be in the same league as Madonna as an artist. 18 years into the music industry and basically 0 growth. How long will she sing about her exes? Her prime will not last forever.