These exist, actually, they're just not really that great or durable (and they're expensive). A guy I know has a tennis-playing father in his 70s who has a cushioned soft-surface court at his house. You can put your thumb to the court and feel the give. It's not a bad court - definitely easier on the body than a regular hard court - but the bounce is really low. Still, it's not a bad court. But I want to say it cost over US$20,000 to install the surface over the original hard court and it develops dead spots over time where the ball doesn't bounce at all. I think there are various companies that make something similar to this. (This is not plexicushion, by the way, which is only slightly softer than concrete.) So, they're out there, but not that popular.