By definition, preventative health care is employed when the financial and morbidity load can be reduced in comparison to active healthcare. Not every screening methods offer such benefits, and thus are not considered part of preventative healthcare.
Many injuries and illnesses will have no advanced symptoms. Preventative healthcare cannot predict things like car accidents. Therefore Reactive healthcare infrastructure is needed no matter what. It will be more expensive to build infrastructure for both methods of healthcare.
How does one know what to prevent. It would be exponentially more expensive to prevent all known diseases instead of reacting to a single known disease, and then I could still die in a car accidet tomorrow.
Preventative health care requires screening the entire population for the off chance a few of them require preventative treatment. Reactive care doesn't require resources being spent on the healthy population. Compare spending $10 on 100 people to prevent spending $900 on one.