I think it is because the aura of "invincibility" exists, especially and maybe more so in younger athletes. They figure that they are fit, or can turn it on any time they want to, and can eat whatever they want, because their metabolism is high. They underestimate the effect of aging. 10+ years later of beer drinking and sitting on their butts, voila- you know what you get.
I have never been a good athlete, but I love sports, so I played tennis (badly) as a teenager, and did every intramural sport I could in college. After grad school, I got married, we had kids, and I got too involved and stressed in my job. I thought I could turn it around any time.
Well, I waited until I was 40, but did get moving again. Glad I did before it was too late. I feel like I am 25 instead of 44.
Best 5k time in a real, timed race: 28:16. Best "unofficial" 5k: 27:59. I have been running for 18 months.