College degrees show that you can stick to working at one thing for four or five years. They show your dedication and motivation, and the knowledge you posses. That puts graduates way, way way above people who are self taught, reguardless if they are just as motivated. Employers like degrees. The more letters after your name, the better.
Web design doesn't need a degree because you're often self employed. Welding, machining, etc, are skills you can learn and get good jobs with, but the pay won't come close to someone who has a degree in mechanical engineering from a reputable school. In some cases the starting salary might be lower, but wait a few years, or if you get a masters degree...a non-degree job will never come close. And with a degree, the longer you work, the more money you make.
Bassoon, n. A brazen instrument into which a fool blows out his brains.