History.
Radios used to blow. I don't mean 'sorta scratchy'...I mean hurricane force blow. It actually hurt your ears trying to understand what the other guy was saying. And we're talking corn bread American to corn bread American. No language issue...just Cat 5 hurricane force blow radio. There was a soft voice down in the side noise and to hear it you had to turn it up...loud! Even then is was broken and scratchy.
So, they came up with the phonetic pronunciation posted above (post 2 or tree) to make the numbers and letters distinguishable. With bad radios you don't want to be asking, "3? No, B. E? No B. Z? No, B. G?" Get the point?
Fast forward.
Radios are pretty darn good now. There's no real need for fife and tree. But now we also have foreign students clumped up in certain areas and that results in its own degradation to comms, so the old school trees and fifes still get used.
Personally I use trees and fifes and niners mostly to be cute now but most here could have figured that...
Break, out.