With legs outstretched, the lever arm is longer and requires less force on your part to raise yourself up off the floor.
Work Step by Step
To do a sit-up, your core muscles exert a torque to rotate your upper body away from the floor. The force of gravity acting on your lower body provides a helping torque that assists the core muscles in that effort. However, when your knees are bent, the lever arm for the helping torque is shortened, making it less effective. Your sit-ups become harder.