You'd expect Silicon Valley working practices to be less optimal than those in mature industries, because, well, the industries aren't mature. The companies are often run by people with minimal management experience, and the companies themselves are too short-lived to develop the kind of institutional memory that would be able to determine whether such policies were good or bad. Heck, most of SV still follows interview practices that have been actively shown to be useless, to the extent that they've been abandoned by the company that originated them (Microsoft). Success is too random for these things to be noticeable; the truth is that in SV, being 50% less efficient probably has negligible effects on your odds of success, because the success or failure of a given company is massively overdetermined (in one direction or the other) by other factors.

The only people in a position to figure this kind of thing out, and then act on that knowledge, are the venture capitalists - and they're a long way removed from the action (and anyone smart has already left the business since it's not a good way of making money). Eventually I'd expect VCs to start insisting that companies adopt 40-hour policies, but it's going to take a long time for the signal to emerge from the noise.