In show they created Mecha before a Plane. Even though they have created plane but still use airship. They have tanks 50 years before any cars. They created a complex drill machine before other thing.
There’s no reason why their technological Roadmap has to follow ours. They have magical abilities.
Our technology development happens out of a series of events and discoveries that could have happened in totally different orders if a few historical figures had died from the plague, or Spanish flu, or measles.
And besides that, war has often been a driver of technological innovation. The earth kingdom has historically been a defense-heavy kingdom, so it’s natural it was the fire nation who focused on air combat.
Big note on that last point: the Fire Nation knew that air combat was a possibility because, up until 100 years prior to the events of ATLA, the Air Nomads existed and were able to fly. We didn’t have that.
