Because it is a horrible, horrible job with no future. If you haven't already matched into an advanced program for next year, a prelim surg position gives you the opportunity to be somebody's 120 hr/wk b1tch for $45K without getting anything in return. In all likelihood, you'll get little or no time off to interview for other positions next year. At the end of the year, they'll probably offer you another prelim year w/o advancing you in the program and you'll be right back where you started.
But if being an intern next year is the only thing that matters to you, go for it.