Anyone (or almost anyone) can come here on a tourist visa and just stay.

To work, they can buy a green card illegally.

Illegals pay taxes if they have fake green cards (because they pay into the account of the person who’s ss# they are using.)
They don’t pay taxes if they get paid in cash, which is what the majority of construction workers are paid in.

I agree with Bugs (always the voice of reason and rational on this forum!) that most of them will go home when the boom ends, and we don’t need them anymore.