Remind me again what's BM's equivalent to fireball?
BM's mobility is unrivaled.
On top of that, they can move during their immunity, mages cannot.
BM has access to a far wider range of utility spells, partly thanks to pets and the BfA revamp.
It is only natural that a spec that has so much to offer besides top tier damage does not have top tier damage. To what extent their damage should be taxed, opinions may differ, but I would wager most people agree BM does not deserve to do as much damage as specs that are nowhere near them on the other metrics.