The big Empire in my setting has some inspiration from the pre-Islamic Persian Empires, and thus they really like Fire. They were the first to develop fire-magic and all of their priests are educated in the art. These mages use their magic primarily for religious rituals, but occasionally also hygienic purposes for the very wealthy (only rarely because it's kinda dangerous). The fire is manipulated via magical energies and it looks like the magi are making the fire move on its own.
This has massive implications for their religion, they see fire as the arbiter of all purity, justice, order, and truth in the world, and the source of all light as well. Thus it is used as a potent symbol for entire empire.
The problem is that I really don't want there to be Fire Breathing warriors in the military, I'd prefer to keep it more low fantasy in terms of tone and I really don't want to make a rehash of the Fire-Nation, but I can't figure out a good reason why they wouldn't include it. Fire is just SO powerful.
I'd also prefer if you try to come up with a more interesting solution then "it's really expensive/hard to train people in it".
Thanks a ton.