You can think about this situation on a much smaller scale, such as in a single household. Imagine one household with 5 members doing great in terms of income and stability, you can't just shovel some outsiders (like an adopted teen) inside it and expect the household to stay the same. It depends on the level of education, culture, character, work ethics, etc. Even if the new member adds some to the income, it doesn't mean that the household's general well-being is better. Even more problematic if the new member brings more problems than benefits.

The problem of the US (and the EU) is that the migrants suck more money than they generate. This is because a lot of them aren't skilled/educated, and some cannot even speak the country's language. They also come as job seekers/unemployed and become the competition for the existing (local) workforce. To avoid this situation, countries have a checklist of who should get a permit, who's gonna be a permanent resident, etc. But the system is destroyed by illegal aliens and asylum seekers. Thus, the mass deportation is an expensive price the government must pay (in the short term) to fix the long-term economic problem.