Because the only time period you can apply for H1 in the US is April. And the result comes out in 2-4 months (not exactly sure how many months). If I graduate in May, and apply for visa in following April, the result would come after my OPT would have expired. To keep myself in "status", I would have to go back to school. But if I graduate in December, the earliest I am anticipating to get a job with my OPT is January. And I am not sure if my employer would be willing to sponsor my H1 based on my past 2-3 months of performance. I hope that explains.