I would guess it's to limit exposure to error. If the voice is recorded, that's one less thing that can be affected by fatigue, sickness, etc. The recorded voice is going to be the same every time. Plus, most of the time I've heard singing in drum corps, it's been somewhere between terrible and decent, but never great. Use a recorded voice and you can get exactly what you want, evey time.
That's my guess, at least.