Logistics. Practicality. 10,000,000 illegal immigrants. Let's say we will deport all of them over the next 10 years, assuming no replacement illegals of that period and they all politely declined to re-enter the country. That's over 2700 deported each day, including weekends. If we as a nation were really really enthusiastic about the task, and citizens readily turned illegals in, maybe we could do it. But we aren't and we can't.