y do matresses have designs if they're covered up by sheets?

Answer 1

It's just to draw the attention of the buyers in the store. If it looks too plain, it'll get overlooked, or a prettier one will get chosen. That's just the way the human mind works. We know that it'll just get covered, but we tend to go for the best looking one anyway. Go figure.

Answer 2

because they're sold without a cover.
it has to look attractive to the buyer.

Answer 3

To hide the c u m stains?

Answer 4

marketing