My grandfather was drafted for the Korean War but instead of being sent to the front lines he was placed in the U.K for mechanic duty even though he has no experience as one, and far as I know there was no physical or mental problems with him. Why was he sent to U.K. to make and build jeeps and tanks when the US was already doing that and the front was low on man power?
The U.S. military maintained a very strong presence in Europe throughout the Korean War. In fact, there were concerns the Soviet Union would exploit American distraction to strengthen its position in Europe or even invade; in December 1950, President Truman sent an additional four divisions to Germany. Over the course of the Korean War, U.S. forces in Europe went from 100,000 deployed to 250,000.
As for your grandfather's specific assignment, one of the first things all draftees did was take an aptitude test which ranked them by percentile score. If his test score indicated a high mechanical aptitude, regardless of experience, he would have been flagged for training and assignment as a mechanic.