Given the sales figures, there seems to be a demand.
Right now, it's just the trend that clubs, or more accurate their kit suppliers, create all the different styles of shirts and training gear.
Celtic might keep the same fundamental design, but there is still a huge demand for their latest version very season, and their second kit, and their third kit, and their new training gear etc.
As I've said in other posts, and I'm not joking, older fans are not the demographic. The majority of fans seem to like a new design every season. I'm not one of them, but not buying them is a pretty easy problem to solve.