Why? The answer is SIMPLE. First off-They usually don't have the time to dedicate to all that this involves! And secondly, they don’t want you to get stuck with an astronomical medical bill by running tests that your insurance may not believe you need! And neither do I! Which is why I have committed to using a third-party Functional Testing Service.
This ensures that even if your insurance refuses to pay- YOU DON’T GET STUCK WITH THE BILL!