Because that’s extremely unnatural.

When I speak to an agent, siri, or whatnot, I am always worried that they will assume I'm done talking when I'm thinking. Sometimes I need a many-seconds pause. Even maybe a minute… For Sire and such, I want to ask something simple "Hey Siri, remind me to call dad tomorrow". Easy. But for Claude and such, I want to go on a long monolog (20s, a minute, multi-minutes).

To me, be the best solution would be semantic + keyword + silence.

Hey Agent, blablablabla, thank you.

Hey Agent, blablablabla, please.

Hey Agent, blablablabla, oops cancel.

And suddenly your address book has changed the name from "Dad" to "Tomorow"

Never skip an opportunity for a dad joke.

I have the same issue. It gives this very weird minor sense of public speaking anxiety where I almost feel the need to write down what I'm about to say, which negates the whole purpose. Only solution I've found is using push-to-talk with some of the system wide STS applications.

"Alexa, what's tomorrow's weather? Over."

"It will be sunny with a high of 10 degrees. Over"

"Thank you. Over and out."

Just add some noise and Push-To-Talk and it will be great for ham radio enthusiasts!