Surely a blind person relies a lot on audio input?

Maybe on a smartphone, but usually not on a computer. Keyboards are pretty good.

The other thing is that if you're around others, voice input means you have no privacy. Even if you're not doing anything particularly private, it's a bit awkward and potentially embarrassing. If you use touch input in conjunction with a screen reader, you can be more like a "normal" user in that what you're doing is just between you and your phone.

Audio input is far more commonly used by people with mobility difficulties. Imagine that your hands shake a lot or that you don't have limbs. That makes using a keyboard and mouse difficult or impossible and voice input can help. Blind users generally use keyboards for input, the typical ones you find on a PC but also sometimes the keys on their Braille display.