Indeed, but it's still a fair question, just framed badly. What he should have said was "who works in every kitchen in every diner, and who maintains every public green space, and who builds every house?"
Other first-world countries get by just fine without an illegal-immigrant underclass to do the menial jobs. In Australia, building houses is a respectable and well-paid trade. Kitchens in fast-food restaurants are worked by high school students with part-time jobs. And public green spaces are maintained by... well I dunno, some low-skilled worker making a decent wage.
We also have a 5.2% unemployment rate, and no real social underclass the way there is in America.
Also, there's an important difference (to me, anyway) between 1) a descriptive statement about who does certain kinds of work in today's US, and 2) a prescriptive statement about who I think ought to be doing those kinds of work, and under what conditions they ought to be doing so.
I've done farm labor. My father did the "follow the harvest" thing for years.
I completely reject the idea that there are jobs that "Americans" won't do.
Maybe you won't do them, but if you won't, why should anyone be taxed to support your choice?