I know a lot of underground rappers can make more from merch sales at a show than they get paid for actually playing. They need that money, it's how they make their living. If you're a major artist getting paid five or six figures to play a show, making more money probably isn't the highest priority.

And since you brought it up, the only major acts in all of music that I see doing well with tour shirts are country music artists. But they're also the only ones still doing well with album sales, so that makes sense.