Any wizard flying around casting fireballs hasBeing led by a guy flying through the air throwing fire at your enemies sounds a lot more inspiring than your leader being just another guy with a sword.
Yes, the queen was a symbol. And in the past nobility went so far as to claim being chosen by god or even being divinity themselves to appear as a symbol.
Imagine how much better that works when they can wield supernatural abilities through spellcasting.
A) used up all their power in less thana minuteif constantlycasting. At the most they get 10 minutes of flying.
B) made themselves a prime target for dozens if not hundreds of archers every round.
C) made themselves a big target for enemy spellcasters with counterspell or dispel magic.
D) Assumes no flying enemies.
Cool for a minute or two, after that suicidal. Also assumes standard foot soldiers find it inspiring, not terrifying.