Because they won't have a passion for detailing. Most are just looking to make a few bucks on the side, as a temporary job so they can later be a hedge fund manager and make millions of $ an hour after they graduate.
They might not take the job seriously. You want someone who is so into detailing that they would do it for free because they love detailing so much. Out of random college students maybe 1 in a million will really want to be a detailer. Most want to party on their time off during their college years. Ask them where they see themselves in 5 years. I'll bet a hedge fund manager's Porsche Cayenne fuel bill it won't be cleaning coffee stains out of cupholders.
:work::autowash::drool:
P.S. Take a look at their personal cars. Is it properly detailed? Ask them who details their car. If they can't be bothered to detail their own cars what makes you think they will be motivated to do someone elses?