Because once you start sending secret cameras and gps trackers across jurisdictions for the purpose of catching criminals you really do need a pi license. If your homespun amature detective agency makes a mistake, or gets someone hurt, you better be ready for a multi-year legal headache. Maybe you win out in the end, but it wont be a fun ride.

Every department store security guard knows to never continue the operation beyond the store. Their insurance companies know the potential troubles and wont cover such things.