I think most MBAs are looking to work for a large company, climb the ladder and so on. The training focuses on that -- it's not called a Masters in Business Administration for the heck of it.
Also, good quantitative skills, and a good understanding of scaling and system behavior are important.
I think another issue could just be that Engineers aren't paid as much as MBAs. Which means we need to take that risk if we want to make the bucks.
Also its generally easier for us to take the risk. If you're an engineer making 120K its easier to take a paycut to 100k if you form a startup. If you're an MBA dude making 500K it just doesn't make sense to give up the Rolls etc and take a paycut to 100K.
Some MBAs make ~140K out of school. EG McKinsey is known to make you work your tail off for very little $ comparatively speaking. But thats a golden stamp: 4 years out of McKinsey and you could be a SVP at some dumbass BigCo in Silicon Valley making ~500K. Why the hell would you want to give it up and join/form a startup?
And, as others have noted, the starting salaries are highly variable -- by school, by industry, and especially by job function. When people speak of "average" MBA salaries, they're usually speaking of a mean figure and not a median -- which is a bit silly, because the mean has traditionally been skewed very heavily by investment bankers.
At the end of the day, it's somewhat intellectually lazy to compare "MBAs" to "Engineers," as if both sets are relatively homogenous, or even normally distributed. The fact is, the designation "MBA" could mean so many different things. It could mean a nonprofit manager earning $65k, a brand manager earning $80k, a consultant earning $120k, or a private equity associate earning $200k+. To a certain extent, the same can be said of the broad category "Engineer" -- although there are so many subspecies of "Engineering" masters degrees, that selecting any one of them can yield a more homogenous set to work with.
In the long run, one thing seems abundantly clear: there are perhaps more MBAs entering the job market each year than the job market will actually bear. Conversely, there is a dearth of engineering talent, and a consequent seller's market in most engineering fields. That means engineering salaries will likely trend higher, while MBA salaries trend lower.
Also with just seeing how things operate in the Valley. I haven't seen McKinsey around a lot here though...which may be a good thing. Back in the tech bust years, they seemed to have just one piece of advice for every problem: outsource to India :-)