Probably because you don’t have to move the joint to work it since your upper body is supported by the hamstring at the hip…if you didn’t hold this position your upper body would just fall over and you would not just be bending at the knee but at the waist as well. Just like Abs, you don’t have to move the muscle to work it, using them in a supporting role is still using them.
Yes, I know that you use them as a support. My point is that anyone who has the knee flexor strength to do a real GHR is not going to be challenged by the moderate stabilization of the hamstring at the hip. You are not providing any overload for the hamstring at the hip and greatly stressing the hamstring as a knee flexor.