Because they talk over noisy, distorted channels that have about 3 kHz bandwidth, in environments that are noisy to begin with?
You can talk to your PC in 16 bit (or more) audio sampled at 48 kHz, with a decent microphone, in a relatively quiet environment. Moreover, it could be tailored to understand your voice specifically, not just anyone saying "U".
In either case, you're using the same number of syllables, so there isn't any speed lost. You just have to spend a day learning the alphabet.
In 2023, if whatever you're using doesn't listen for stops, here is a dime, get yourself a better speech recognizer?
I'm not learning any phonetic alphabet other than the ICAO standard one.
If I ever have to use that one, I will look like a fool if that nonstandard one starts slipping in by mistake.