One reason I’m not seeing here is that big companies have such high communication overhead. It’s incredibly hard to get large groups of people working efficiently toward common objectives. You can choose between heavy bottlenecks or wasteful duplication of effort, and there is constant churn trying to balance the two. Just knowing what is going on and who to talk to becomes important, hard work. Informing other people what you are doing, why, and how it’s going starts to take significant time. On a small team you’ll never even see how much of a challenge this is.