All intellectual activities are hard to observe and monitor. An idea that is 80% complete has no physical manifestation. It's an idea, and it's not done yet. Sometimes we have experiments or proof-of-concept code or notes, but they don't give an accurate "% complete" number as does physical work.

A chair being manufactured looks about 50% done at the 50% mark. When it's done, it looks done.

A design for a chair may not exist on paper until it is more than 70% complete. And we don't know that it's really 70% done, because it's not finished being designed yet.