Ingredient prep is intentionally left out of recipe timings. In theory, this lets everybody adapt the recipe timing to themselves, because only you know if you can chop two onions in three minutes or thirty seconds, but as with the Doomsday Device in Doctor Strangelove, it's worse than useless if nobody knows about it. "Why didn't you tell the world?"

I very much doubt that most recipe timings actually involve a timed test cook at all. They're just guesstimates.

I imagine it’s done that way because people don’t necessarily know what to look for when cooking something. If some one doesn’t know how to cook steak, you can’t tell them “cook till medium rare”. So we start trying to come up with proxies.