What is one factor that prevents a pilot from selecting a V1 less than VMCG?

Selecting a V1 less than VMCG is critical for safety during the takeoff phase of flight. VMCG is the minimum control speed on the ground, which is the lowest speed at which a pilot can maintain directional control during an engine failure scenario. If V1 (the decision speed) is set below VMCG, the pilot could lose the ability to maintain control over the aircraft if an engine fails prior to reaching this speed, leading to a potential loss of control on the runway.

The essence of maintaining a properly set V1 in relation to VMCG is to ensure that, in the event of an engine failure, the pilot still has sufficient control authority to keep the aircraft on the runway centerline. Setting V1 below this critical speed compromises the pilot's ability to manage the aircraft effectively, thereby posing a significant safety risk. This is why maintaining V1 at or above VMCG is a fundamental principle in aircraft performance and safety during takeoff.