That's because most American have mortgages, car payments, and other bills to pay and working one or two days a week as a day laborer won't bring in enough money to pay their bills. On the other hand, with many Hispanic day laborers living 10-12 in an apartment or 30-40 in a house, their expenses are relatively low so they can survive financially by working only a couple days a week.
Well, except for those of us who left the patrón behind a few generations ago, got educated, then got too "uppity" for bayourod's taste, making it necessary for him to import illegals.
Then work 7 days a week as as day laborer. What do these people do on those days when it isn't worth their time to be day laborers, sit home watching Gilligan Isaland reruns?
At least you are admitting that the day labor jobs are ones that Americans don't want to do. Since it isn't worth Americans' time to do them why do you object to immigrants doing them?