If you're ever wondering why something in the U.S. can't be made better, the answer is often that the existing rent-collectors must be allowed to continue collecting their rent at all costs. This is why we can't have the government file taxes on our behalf: because Intuit would no longer be collecting rent, and rent-collecting must be protected at all costs. This is why we can't have a single-payer healthcare system: because insurance companies must keep collecting their rent. This is why the TSA is here to stay: it collects rent from the taxpayers, the airports, and the passengers.
Any modification to the system must include ways for the rent-collectors to continue collecting rent from someone. TSA Precheck is a way for you to say: it's fine, TSA, I'll pay rent directly to you, so that I can avoid the worst parts of your services. It, of course, has nothing to do with actually pre-checking that certain passenger are low-risk -- it's just a solution to the problem of "but if we made the system more efficient, how would we still collect rent?" Collect it directly from the passengers themselves, for the privilege of avoiding the worst parts of dealing with you.
Any modification to the system must include ways for the rent-collectors to continue collecting rent from someone. TSA Precheck is a way for you to say: it's fine, TSA, I'll pay rent directly to you, so that I can avoid the worst parts of your services. It, of course, has nothing to do with actually pre-checking that certain passenger are low-risk -- it's just a solution to the problem of "but if we made the system more efficient, how would we still collect rent?" Collect it directly from the passengers themselves, for the privilege of avoiding the worst parts of dealing with you.