Why can't the OCR1A register be set to 1 to get a 8MHz PWM? The max PWM frequency that can be achieved with an Arduino is by using the PWM fast mode with the pre-scaler set at 1 and the TOP counter value at 3, getting 4MHz 

f_OC = 16000000/(1(1 + 3)) = 4000000

Why can't be the TOP value set at 1?
Because then you only have 1-bit PWM. And there aren't an awful lot of applications for 1-bit PWM.