Why do doctors and other health professionals withold abnormal test results when it could cause other health problems in the future?

They don’t, usually.

Test results have different sensitivity and specificity levels. Having one or even a few tests that are too high or too low does not necessarily mean that something is wrong. Occasionally the tests are wrong. Other times it is just a coincidence and was just abnormal that day. Sometimes it is just the level it is at with a patient and it will not cause them any harm. A physician's job is to look at the overall clinical picture and figure out the diagnosis. Lab values are a component of that picture but they do not represent the entirety of it. Many doctors do not show patients the test values themselves as patients often panic when they see labs that are flagged as too high or low. Even when they know that an abnormal value is not always cause for concern it is hard for many to not focus on it.

If a doctor does notice a lab value that is cause for concern, it is their legal responsibility to act on it or refer you to a physician who can better treat the condition. It is highly unlikely that they would see something wrong and choose to ignore it as that would open them up to major lawsuits and potentially the loss of their license.

Doctors do not withhold abnormal test results. It's against their code of ethics. If they do, they can lose their license to practice medicine.