https://twitter.com/SimGuruGrant/status/1001874416593063936
From this tweet it seems that they actually tried to research it and tried to implement it on the game, and it turns out ugly. They might already find a way to implement it but then again its was visually unappealing. They knew what happen about the ball pit so they don't want to do it again, and it seems that its better to have them flat that looks beautiful rather than have them yet it looks ugly and dosen't fit in at the art style.