I don't live in the US so tipping to me is a bit of a novelty (we have service charges). But as a visitor I thought I got it and it was quite straightforward - albeit slightly illogical.
And when I question about the logic I get shouted down. I am told its established; everyone accepts it; its deeply ingrained in the culture; it drives great service; without it service would be dire etc etc.
But there are six threads in the first 25 on this page today asking about tipping. Why...?