“But they build the estimates on facts. You take the existing data, facts, and extrapolate them to estimates.”
If you don’t think there are any assumptions built in to that process, well, I’ll just let you go on thinking that, if it makes you happy.
There are assumptions built into everything, but the problem here is that you are assuming that assumptions are their sole source. That’s just silly. The concept is simple: take known facts from the period, take known facts about general trends (there’s the big assumption, that other trends apply), build estimates from there. The starting point is still always FACTS not assumptions. But you go ahead and keep assuming if it makes you happy.