the thing is, management is accountable to a budget, timeline, and an ROI. They have to be able to say how much something is going to cost, how long, and how much they're going to get in return. Now, a person doing the estimation has to be able to take everything into account, down to the cost of context switching, but those three things are all that matter upstream. Also, the people doing the estimations have to be able to answer questions like "well, what can you do in, say, half the time?". It's not easy.