I disagree with the overall premise, in the U.S., at least.
A BS typically takes four years (eight full semesters) to complete.
1. You can shorten that somewhat by taking classes in the summer, but you wouldn't graduate an entire year early (May of your third year). You would finish after your third summer (August of your third year). So you might end up starting work eight or nine months early, not an entire year early.
2. Also, not every required class is offered during the summer. Summers may offer intro and intermediate classes, but likely not the most advanced classes in your major. And you can't take advanced classes until you've taken introductory and intermediate classes, so it's not as if you could take them in the fall and spring of your third year and save the intro and gen-ed classes for the last summer.
3. Financial aid doesn't cover summer semesters. You -might- get a refund check if your financial aid exceeds your tuition (plus room and board), but you can't really count on that happening every year. So you'd have to come up with the money to pay for summer classes somewhere else. Students with the wherewithal to pay for summer classes don't have to worry about starting to pay off loans a few months early, and students without the money would have to take out additional loans to take summer classes, which completely defeats the purpose.
4. The other option is to overload your schedule, but most people find 15 credits per semester more than enough to keep them busy. Grades will suffer if you take on too much at once, and, again, you still wouldn't be finished after six full semesters, so you'd graduate half a year early at most.
5. You can't work a part time job or take an externship during the spring and fall semesters if you've overloaded your schedule, or work in a full-time internship during the summers if you are taking classes full time. So while you may graduate early, you don't have the work experience employers prefer even in entry-level applicants, and you haven't made the contacts and networking opportunities as you would have with internships. (I was hired full-time at the organization at which I had an externship in law school, and at my last job, I worked with someone who was friends with my boss at my part-time job in college (small world!)). It's better to build a decent resume in college than to rush your way through and not qualify for any decent jobs at the end.