It's not "unskilled" so much as (a) it can be learned on the job and (b) people don't see why they should pay more for top talent.
The second one is pretty important. Consider that sales, sports, and entertainment work similarly, where you can make a case that you should pay more to get the best and workers (or their agents) can use this to good effect.
I think "unskilled" and "can be learned on the job" are usually considered equivalent. Nearly anything you need to hire a human to do requires some kind of learning, at least to do "well" or "our way".
