I recently read about several heart disorders that can go undiagnosed (such as long qt syndrome) and can cause sudden death. why aren't all people screened for this?

All ?????????: If during a regular checkup your family history reveals any relative with sudden or unexplained death they would be. Rare things rarely happen. We don't do whole body MRI's and daily blood tests either because the pickup rate for a treatable condition would be so small, the cost per pickup is prohibitive. The waste of personnel & equipment time would divert them from patients with real need.