The same reason pilots say "fower" instead of "four" and soldiers "foxtrot" instead of "eff" - a good spelling alphabet is much clearer and ambiguity-free than English letter names. Getting into speculation here, I expect the reason they use their own alphabet instead of something like the NATO phonetic alphabet is a matter of use case: unlike radio communications, transcription provides immediate, unambiguous feedback (the letters show up on your screen) and mistakes are low-impact and easily corrected, so a degree of clarity is sacrificed in exchange for improved speed.

bee cee dee eee gee tee vee zee