> [For many problems, a behemoth company has no reason to improve, and a lean startup can rush to a solution faster in many cases.
This is 50% of the thesis of The Innovator's Dilemma, the book that introduced MBAs to the the word "disruption".
The other half if why this happens to behemoth companies, which is that they are listening to their customers. In this case, the customers are those who are OK with waiting 3 weeks and would just like a wider selection or lower prices. People who would but if delivery were faster by definition aren't customers.
Startups have to address the non-customers because otherwise they'd have to pry them away from their existing supplier -- generally a much tougher task.