Because 1. You don’t have enough workers that will do these menial jobs and not do them the way these workers are 2. A lot of the Americans who are unemployed honestly deal with a host of other issues like addiction, criminality, etc that automatically disqualifies them from being reliable workers to fill these roles.
I’m not saying that there are not any who can take these jobs but even if say, for every 100 undocumented aliens who are gone, you have 20 Americans come in to fill those positions, the 80 positions that are left unfilled or poorly filled still debilitate those industries.
Construction, agriculture, logistics and the restaurant industries would all basically come to a near standstill if all undocumented workers are gone.