I think its a combination of
1) Tennis being more endurance-based
2) A lot of 30-something players are not retiring and sticking around picking up points
3) This crop of teenage players is just kinda bad
yeah something like that.
adding to 2)
medical science is much better these days and many players are much more responsible with training, diet, rest and smart scheduling. That means the decline can be pushed forward many years for most players