Because it doesn't always work. I had someone try to get me fired because she came into my office and said "do you have a minute?" and I said (because I was legitimately busy with something important) "No".

She was enraged that I didn't drop everything I was doing to listen to whatever problem she had, and went to complain to my manager, blithering about rudeness, and me not being a "team player".

Since my manger was the one who had assigned me the important work, he told her in polite manager-speak that I had done the right thing and that she should try again later if she still needed my help.