Unfortunately, most of the above responses are true and the challenge for CC to accurately guess the competence of pax to do the deed is a tough one.
The weight of the door is why some a/c now have a hinged overwing door that only requires to be unlocked and pushed - it will then swing up and stay on support struts. This example (if link works) shows a 737-800 on the ground with them open.
Photos: Boeing 737-832 Aircraft Pictures | Airliners.net