Time is rarely the limiting resource for a startup - attention is. Students (at least ones who want to somewhat keep up with their schoolwork) do not have more disposable attention than most working people. As a student, you need to balance your attention between 4-5 different courses, all of which have their own deadlines and assignments and new concepts to learn, and you'll probably be distracted by parties and girls and campus events and a social life. As an entry-level worker, you will probably be given a well-defined task to do and can focus on it, and once you go home, you don't need to worry about it.

I found it much easier to launch side projects once I got out of school, because I could leave my work at work and didn't really care about it anyway. I don't think I reached a level of distractedness equal to my college career until I made senior engineer (and hence have everyone asking me for advice) and started dating, 5 years later.