In the US, this problem starts with high school coaches who refuse to train kid appropriately out of some fear that "they won't have room to improve in college".
Kids matriculate to college undertrained, begin an overtraining/injury cycle. Luckily, the college system provides full-time treatment options, rehab, trainers, etc., so athletes can band-aid themselves together to run a few good races/respectable PRs that indicate the potential for marathon success.
Kids graduate from college and look forward to the freedom and time that adulthood provides. They continue to overtrain for their marathon, realizing how good they had it in college with teammates to train with daily and full-time support system, all while balancing the beginning of a career and the social opportunities of young adulthood. They blow up in their first marathon, blame the race on poor nutrition, and decide to drop back down in distance.
Without the college structure and teammates to help keep the speed in place, the attempts at lower distance PRs come up short, leading to the desire to go back to the marathon distance. This cycle repeats, runner ages, and those PRs set at age 21-23 turn into unreachable goals, but the runner doesn't accept this, and the Peter Pan Sydnrome is born.