An MBA basically teaches you how to apply rational analysis and best practices to the running of an existing business unit. This works great during normal times, when your company has a competitive advantage, you know what it is, you have a large and growing customer base, and you are making a profit on every transaction. It works terribly when the default outcome, if you follow all the best practices and do everything you're supposed to, is that you'll fail miserably. This is the situation most founders are in - if their market opportunity were obvious to someone with an MBA, then someone with an MBA working for a big company with lots of resources would already have done it.