First, simply because aging is by definition “non-specific” degeneration. There is no single cause of aging (or degenerative disease for that matter). Like degenerative disease, aging is a wide river fed by many different streams (sources). Any degenerative disease develops over time by feeding off of this wide river of nonspecific degeneration.
Second, because we develop a degenerative disease by doing something wrong, such as chronically inadequate nutrition coupled with excess toxicity and calories. The more in the wrong we are, the faster the disease develops. A fifteen year old who gets diabetes type II is an extreme case of that. The older we get, the greater the non-specific damage and the longer we have been doing something wrong, the greater the damage to the critical bodily structures, the greater the loss of function, the greater the chances of getting a degenerative disease.
Bad genes increase the probability of getting a particular degenerative disease. They do not cause the problem; otherwise, one would develop the disease at birth or shortly after birth.