Again, 5 hours at Gallagher McDonald's and they could clean up.
Do that country wide and govnt start hitting the companies with huge fines.
In reality that wouldn't work. Those companies have a the system set up so the workers are at arms length. The illegal workers will 'rent' logins to those services from people who are registered legally. Makes it very difficult to take any meaningful action against the companies themselves.
So what do you do? Round up the people at Gallagher and then what, you're not going to be automatically deporting them under current laws so you basically tell them not to do it again and they're back at it the next day because its the only way they can earn money.
If there was a sizeable and publicised boycott of those companies they'd soon find ways to bring in stricter checks on who is doing the work.