I agree that kids are exercising less but I think declining numbers in road races are due to the fact that kids participate enough for their teams. Very few of my athletes have run in a road race over the last few years. Some of my athletes did while in middle school but there is no good time for it when they hit high school.
They are competing in XC right up until December and then they jump right into indoor track the next week. They get a month break or less break going into outdoor during a time where there are very few races. Then after June we are base building again for XC. It's not really a great time to compete in an outrageously expensive and meaningless road race.