DDR generational transitions are curious. It's not like the manufacturing process of memory ICs gets worse.
It happens because people compare enthusiast DDRn with conservatively timed DDRn+1 that gets targeted at server/workstation customers first. The early adopters of a new DDR standard care about system reliability far more than performance, and conservatively timed DDR5 (which is all there is now) is a nice performance bump over conservatively timed DDR4.