I'm not sure that's true. A physical button only works if I'm near that button. Alexa only works if I'm near a microphone (and comfortable with that microphone). Whereas my smartphone is always in my pocket, and therefore always useful.
That's not to say a button isn't also useful - it's usually more convenient if I'm already next to that button, which makes it great for turning room lights on and off if I'm moving from room to room. But if I'm on the sofa and I want to dim the lights without having to get up, the smartphone, for me, seems like the most practical choice.
I suspect what seems like stupidity to you may often be people catering to those with different desires and needs. Just because a smartphone isn't a useful way of controlling lights for you, doesn't mean that it's a stupid design decision for everyone.