"most people underestimate the impact of transaction costs"I think most companies underestimate the impact of transaction costs, but most managers overestimate them. Both problems result from a failure to see the issue played out over the long term. In both cases, people or entities only consider the immediate to near-term impact of the delegation.
A company will look at the present value of outsourcing, which is often highly positive, but underestimate the opportunity costs over a longer horizon (lost institutional knowledge, higher marginal transaction costs, commodification, etc.).
A manager/individual will be hesitant to delegate because he or she only looks at the immediate hassle of having to train someone and get them up to speed. "It'll take Bob six months to really understand this track of work. It'll just be quicker if I get this done." In this case, the manager fails to understand that delegation has high short-term costs but low long-term costs. The learning "curve" is more of a step function. Once Bob is fully competent at Task X, delegating Task X to Bob will create efficiencies for his manager. It's starting Bob off on the task that's the hard part. His manager basically has to invest efficiency: "spending" some efficiency now in order to realize a net gain in efficiency later on.