The main reason is because most "preventative care" does NOT reduce your chances of getting sick and does NOT reduce the cost of care. In fact, it increases the cost of care.
I know what your are saying, but we know about a number of risk factors that increase your chance of needing care. For instance we know that Obesity causes the early onset of all kinds of ailments, such as Diabetes and Haeart disese which require lots of treatment. In my experience, the only health related question that gets asked is "do you smoke" so, a big fatty pays the same premium as a very fit individual. Moreover, with today's actuarial techniques, they are able to to a startlingly accurate cost/benefit analysis.
Ever since I haven't been eligible for my parent's health insurance I have been insured in one way or another though my employer or my wife's employer. They are definitely making some money on me, as I have no chronic conditions that need prescriptions, and the only Dr. appointments that I have had since 1998 have been physicals to clear me for overseas assignments.