I recently read that you can have congenital heart disease and not know it. why aren't all people screened for this?

They are: Very bad CHD will present with life threatening symptoms in early infancy if not immediately after delivery. Some defects like the patent foramen ovale are considered variants of normal & are an incidental finding on an autopsy. The normal process of exam & follow up will find most. It would not be sane to do heart catheterizations on everyone to find a few with minor defects.

Population screening: While congenital heart disease is the most common birth abnormality, the absolute number of people born with congenital heart disease is small compared to whole population. Therefore to screen the whole population in the absence of symptoms does not make sense to find few potential patients. If one is not feeling well, or has complaints, then congenital heart disease may be considered.

Complicated: Screening only makes sense if you have a great screening test and disease is common. Many tests have false positives, which can ultimately cause harm to patients by leading to unnecessary tests (not to mention anxiety and cost). Nearly all 50 states do screen newborns for critical congenital heart disease (i.e. the most deadly types). But we do not have great screening tests for all types of CHD.