Estimates are hard because you are literally making something that has never been made before. Yes, you have experience with sub parts and similar patterns but

Estimates are hard because you are literally making something that has never been made before. Yes, you have experience with sub parts and similar patterns but not the situation of this timeline, this team, this technical debt, etc. Too often the request is along the lines of "How long does it take to build a house?" not "How long will it take to build the house in these blueprints with these systems on top of a mountain that is also impervious to mudslides?". People can generally estimate the former but the latter is all about the unknown details.