1. Taxes.
2. Because Mexicans will work and Native born Americans bitch about their benefits or having Saturday off (ever been to the grocery store? What are they talking about? Right, their work schedules, every single time).
3. Getting around red tape. E.g. in Massachusetts, the states says that I have to buy insurance for employees. Guess what employers are going to do? Right, hire illegals and not have to pay.
Government creates a situation where compliance with the law is costly and darn near impossible. It then reacts in shock when hardworking, creative people find ways around their inane laws.