What about this. You select a V1 lesser than VMCG.Than during your take off roll exactly between V1 (that means that now you are comitted to go) and VMCG you loose one engine. Guess what is the next thing that's gonna happen
Do you think you would be still able to keep directional control and continue the take off. I don't think so.
If this is clear to you then you should be able to understand why V1 must be equal or greater than VMCG