The permanent magnets(*) spin round with the fan, replacing them with electromagnets would require slip-rings....
(*) often in these small fan motors the magnets are actually a single strip of rubberised ferrite material inside the rotor bell
and magnetised with a repeating pattern of N-S-N-S poles.
The whole point of a brushless motor is to avoid the problems with having to conduct current to a spinning armature
via slip-ring or commutator - one less thing to wear out.
