1. You can't just press a button on your phone to instantly start an ai voice conversation
2. You can't bring your smart speakers with you everywhere, also same as 1
3. See 1 and 2
You absolutely can just press a button on a phone to instantly start an AI voice conversation. Heck, you don't even need to press a button in a lot of cases.
Yes you can. Many phones have a dedicated "assistant" button now that you can assign to your AI assistant app.