Fred is starting to headline some festivals this summer, probably due to the absence of headliners and because he is growing a lot. Totally deserved. But they are mainstream festivals.
On the other hand, Garrix only goes to electronic music festivals, he is not the typical artist who performs at Mad Cool, Primavera, Pukkelpop or Roskilde, places where Fred can play. (with the exception of Sziget, which has an EDM headliner every year).
If Fred Again were to perform at tomorrowland, where Garrix has been a headliner year after year, Fred wouldn't go above him.