The metal inside the socket that completes the circuit with the male pins can age and get bent, losing contact unless the plug is at just the right angle or the pins are bent in the right direction. It's unreliable and unsafe. If its your house and you're intimately familiar with the state of the sockets, it's slightly less unreliable and unsafe.

The EM coming off AC mains makes a non contact detector hard to fool and it's such a simple device that even cheap knockoffs are reliable.