There is no single reason. It’s the sum of many reasons. They’re too many to list exhaustively but when we see a concrete example the vast majority of people come to the same conclusion on creepy vs appropriate.
When there isn’t a clear line, trying to define one is misleading. You can always find some couple somewhere on earth with an arbitrarily large age gap where people will agree that it’s the result of informed consent. People then try to make the argument that this justifies all relationships with that age gap even though most relationships don’t have whatever extenuating circumstances made the one example palatable.
Large age gaps are creepy. Whenever someone has to ask if a particular age gap is also creepy the answer is almost always, “Yes.”