To propel the craft straight to the moon you 'd have to time the launch perfectly. You then need to know that rockets don't fly straight up. They only go straight at the start of their launch to get out of the thickest part of the atmosphere, after which they curve out and fly more horizontally. You then need to manage resources, the craft can only carry so much fuel and you have a return journey to make, far better to simply maintain momentum and use earth's gravitational pull to turn around rather than attempt to expend fuel generating thrust to counteract the momentum you already have to change course and go straight to the moon.
