This is hardly unique to USA, the supermarkets here in the Netherlands also offer sliced mushrooms. I never buy them, but find it hard to judge if well off people want to spend an extra 90 cents to not have to spend 3 minutes cutting them.
I've heard sliced stuff is something desirable for people with some disabilities but I am pretty sure most people who buy these kind of stuff don't suffer from any of them.
I am pretty sure vegetables/fruits will get bad and loose their vitamins faster too if sliced.
Also it is a waste of packaging for nothing. I'd rather have someone standing in the vegetable area of the shop to slice stuff for the handful of people with disabilities than have them already packaged.
I looked up the price difference locally on Instacart. $1.65 for 8 oz mushrooms, $1.75 for 8 oz sliced. The "sliced" version is labeled a "best seller".