Presumably because interactimg with the keyboard is smooth and quick.
A keyboard is also private and works in noisy environments. Some people (including me) would prefer not to speak when avoidable, jusr as some would prefer not to type when avoidable. It may be easier (or higher throughput) to use audio feedback from the computer while providing touch inputs than if both feedback and input are audio.
Personally, while I've spent lots of time training typing, the learning curve on speech to text is too high and the apparenr reward too meager for me to go much beyond the bare minimum (I can cancel many accidental summonings of speech interfaces)
I just broke my screen a few days ago, and b/c of the lock-down here (im in a tiny city) everything was closed. So i was forced to use audio inputs as much as possible, and on the IOS i was horrified that i couldnt simply say "Siri, close this modal" or "Siri, send new whatsapp text to Jane"... shocked.