It's just that demanding someone's attention without expending the effort to be interesting to them is quite rude as well. If someone's constantly on their phone while you're speaking to them, your conversation is clearly not very interesting to them so you should do better.

I find demanding someone's attention more rude than not giving attention that's requested. Someone's attention is theirs to give, not something for others to demand.

As for how conversations used to work... When people were boring or uninteresting people would zone out and just uh-hu away. Just smile and nod. It may give the other the illusion that they're not being ignored, but they are.