It's because too many people are too lazy to call ahead of time. Or, they think they need it done immediately.
You are absolutely right. It's the need for instant gratification when it comes to labor. Instead of having the courtesy to arrange for an American citizen to do the work by calling a day or two ahead of time, those who hire illegals wait to the last minute.
They also seem to not even think of them as people. The one time I had the misfortune of being at a home where someone had hired illegal day labor, I was sickened by the neocolonial attitude. Able-bodied people were standing around "supervising," expecting the laborers to work far harder than they would have, and the person who hired them seemed to not want them to even catch their breath because the person was concerned about getting their "money's worth." One of the laborers had Central American gang tattoos, but again the person who had hired them seemed oblivious to the idea that they may pose some danger by knowing where the person lived, who lived there, and what items that person had in her home. I jumped in and started working alongside them, which I think shocked the laborers, the able-bodied "supervisors" and the "employer." But since I had arrived too late to prevent the foolish, lawbreaking move of hiring them, I figured I would do my best to make sure they were sent on their way as soon as possible.