It's the same reason we're seeing so much strife in the US. In the US, societal opinion and legislative action hardly follow the same path. In fact, IIRC, public opinion has almost no impact on legislation. Wonder why everyone here feels like we're going down the wrong track?
Wishful thinking is a temptation, but it doesn't lead to reasonable analysis. Xi is in fine health at the age of 66. His father lived to the ripe age of 88. His mother is still alive, at 93. We can expect him to receive better medical care than they did, since he is leader of China and they spent several decades getting purged, imprisoned, overworked, and starved by the party. If the Chinese people look for a change in leadership, they had best look somewhere besides the calendar.
Besides, if current dystopic trends continue, most of us won't have 40 years to wait for the expiration of our despotic sovereign overlords...