The "prep time" is _not_ the time for the "prep step"? Internet seems to agree with you, it's wild. Why is it not called "assembly time" or something?
Plus, it's not like that recipes on the internet (or books) are usually targeted at professional kitchens. Or like professional kitchens will take times at face value (and won't test it).
It is a bit weird but not moreso than, say, the word "butterfly". People agreed on one convention and one phrasing and stuck with it and it would be confusing to have wildly different time^pro and time^home scales. Just like pasta tells you the time starting with boiling water, so "5 minute pasta" does not mean "5 minutes from entering the kitchen".
Plus, it's not like that recipes on the internet (or books) are usually targeted at professional kitchens. Or like professional kitchens will take times at face value (and won't test it).