It's not that pop singers aren't talented. It's just that pop music as a genre is pretty much boring, and the artists that appear every season, or every "America's got talent" edition, they all are easily forgettable. You could switch covers and sell Selena Gomez album as Ariana Grande album and noone would even notice the difference.

Pop music is a commodity, and as any commodity it's pricing/value is basically a race to bottom.