Then why are so many so inefficient? In the Silicon Valley mythology, one of the core tenants is that startups can act more efficiently than big companies simply due to focus & size.

If this were inherently true to any reasonable degree, I actually think we would have a differently shaped workforce. I can point out dozens of inefficiencies I experience even today that slow down development, and that is just if we analyze development. I hear from other wings of the company[0] of other things - simple things like consolidating tools so they don't have to bounce between different apps to do things - that also drive alot of real inefficiency.

This isn't the first place I worked where all these issues exist either, and its not nitpicky stuff, its all low hanging fruit. Consolidating tools is a complaint I have heard at almost every job I've had sans 1. I don't imagine I'm alone, my peers at other companies often have similar complaints when we talk.

I think some things are inherent to simply organizing humans, and I get that, but the obviousness of some improvements that simply aren't undertaken runs counter to the assumptions one makes when they buy into the "efficient business" hypothesis

[0]: we successfully became more cross functional, that's a huge plus, our solutions are much more holistic now, but its only part of the problem.