Just no good reason. No reason why someone would actually need one. It's like an automatic lift-gate motor in an SUV. It's not more convenient. Pressing a button to close your trunk is only more convenient than manually closing your trunk if you have a physical disability. And in the long term, it's less convenient: you actually don't have the option of closing it manually removed since this is bad for the motor. If the motor fails in the wrong position, then your trunk may be stuck open. Replacing an old-fashioned piston on a trunk is cheap an easy. Replacing an internal motor is much more expensive and is beyond the skillset of most people.

A voice assistant is similar in an analogous way. A button works every time you press it, but a voice assistant works very sporadically. It's hardly convenient to repeat yourself frequently and randomly. A voice assistant requires internet access, which wastes your data (again, when you could have just pressed a button) and the quality of the voice assistant will change over time. It's less consistent, more expensive, and more complex. And for what? So you can use your phone more often? You already use your phone too much.