In addition to the other reply, they actually don’t catch everything. Metal detectors only catch items of sufficient size, and the only body scan that would be adequate would be a full body x-ray, which would both pose a significant health risk (especially over multiple admissions) and which would require you to wait in a smaller and less comfortable holding area for several hours until it is fully reviewed. An airline (for instance) justifies this increased risk by

a) the passengers having a relatively lower risk of violence, self injury, and confusion and greater ability to comprehend and avoid consequences

b) careful screening / background checks

c) if those background checks result in a high risk, they can refuse the passenger altogether. We are only allowed to refuse patients if we are a low acuity unit or if the patient has too many medical comorbidities / co-occuring conditions.