Why does the quality of storebought pitted fruit decline during summer?

Peaches, nectarines and plums from the supermarket seem to be at their best in early June, just after they first appear. However, over the course of that month they become increasingly mushy and less tart. By midsummer the things are nauseating to bite into, though they often still look deceptively fresh on the outside. What is the reason for this remarkable decrease in the taste quality of pitted fruit?