Because DS does, all the time. I tell/ask him to do something and he just stares at me and doesn't do it, or carries on with his conversation or whatever he was doing before as though it's not important enough for him to even acknowledge. It's not that he didn't hear me, because it happens when I have his full attention. It's just pointless things. "Come on then, get your shoes on" "Mummy look at this fish on my picture".
The other day I asked him to stop waving a pizza slice over my important work documents and he just looked at me and didn't move. Or he says Why. Why. Why. Fucking hell, just do it. He knows exactly why I don't want pizza grease all over my work stuff. I don't mind him asking why if he genuinely didn't know.
He is five if that makes a difference. I just sort of end up repeating it louder until I am shouting he does it. Or sort of steer him in the right direction. But mostly I ask/tell him, he doesn't do it, I stand there at a loss going "WTF am I supposed to do now?"
I'm sure this is in How To Talk, which I must re-read, but just wanted to know what other actual people do, not a book.