Mantraps or similar significantly slowdown the flow, which is an inconvenience. Only big buildings or plants with strict security/safety constrains invest in multiple mantraps at entrances.
Anything else - or even proper poorly done mantraps that can be gamed - are imperfect it is understood that this feature of the access control system is a support feature, not a guarantee.
As I mentioned, a disaster can compromise the working of the system directly, and people may forcefully bypass restrictions (e.g. get out by windows), so you are correct that it only tells who might still be inside. However coupled with other features (e.g. marking users that are on vacation and therefore who are not allowed to be there) can also tell you who cannot be inside.
