I think compare with some Asian countries, Japan is a paradise in job opportunities and salary. To apply employment visa through the proper channel takes time. In fact, some applicants may not be properly trained for the job they apply which make things difficult.
So the shortest cut is to enter as visitor with an intention of taking up employment illegally. Some work as domestic helpers and some work in the bars. Until they are caught and deported, they have no intention to leave Japan.
Furthermore, deportees never stop thinking of going back to their paradise. Some by way of marrying to a Japanese and some use a passport with another name to gain entry. However, neither of them would guarantee permanent stay because the adverse record with fingerprints is always there.
To allow you entry after the 5 years ban period is not compulsory but discretional. It depends whether you can satisfy the immigration officer at that time that you are a genuine visitor and would not overstay again.