If by "brilliant engineers" you mean, finding new ways to display ads and bill people for displaying ads, which is what counts for "brilliant engineering" in Silicon Valley these days.
Putting aside the fact that Silicon Valley is doing a lot more than whatever makes the front page of TechCrunch today, there's still the fact that in order to get an ad impression you have to provide something that an actual person somewhere ostensibly finds useful. At it's worst this is orders of magnitude more useful than Wall Street's endless arbitrage seeking to benefit the richest of the rich without actually providing anything that is useful to someone with less than 10M dollars.
AdTech is a broken industry with most participants providing zero to negative value to the world. They are basically just nodes in a messy, unregulated, scam-ridden chain of players moving an ad from technically illiterate buy side to an equally illiterate sell side, taking something like 70% of the money in the process. Zero value for humanity, no comparison to Wall Street...
But Silicon Valley !== AdTech.
Journalism is in a tailspin because the eyeballs they used to sell for a dollar are worth two cents now. There's a lot of good things that used to be free with ads support - if an engineer can work on a project that gets them the same money they used to, they've done a lot of good.
Yes, Harvard MBAs suddenly become "brilliant engineers" when they take the Google offer vs the Goldman one :)
I think the signal here is that Harvard MBAs are great monetizers and optimizers. They go to Wall Street to buy companies, optimize them (operational or financial improvements) and then sell them. They don't go to Google to program a better search, they go there to help Google capture the value of the search.