Genes as well as survivorship bias. On a related note, numerous seasoned athletes or fitness enthusiasts seem to "break" very fast as they age. While they probably boast excellent cardiovascular health, if the package includes worn-out joints and persistent issues like back pain opting for moderate physical activity appears to be a wiser choice compared to pushing the limits with sub-elite levels of exertion.
People are too quick to blame "bad genes" for their poor results. In reality this is seldom a major factor and they usually can't point to a specific genetic test result. While genetics might play a role at the elite and professional levels there is no real evidence that it matters for masters athletes. Good habits and training are what count.
There is no evidence that participating in sports wears out joints. Joints evolved to move. The majority of people who need joint surgery lead sedentary lifestyles. Building strong muscles helps to stabilize joints and reduces the risk of injury.
