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 radiation health risk as most medically necessary x-rays are specifically limited to only the necessary area (and especially if the patient is admitted multiple times) and which would require you to wait in a smaller and less comfortable holding area for many hours until it is fully reviewed (even a small medically necessary x-ray can take several hours to be examined). An airline (for instance) justifies the increased risk of not doing a full body x-ray 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 where a high risk result means they can just 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.