Why do some enterprises show salary of 4000-6000 when recruiting?

This is the routine of recruitment. Let's recruit you first. After a little time, I felt inappropriate and quit. Generally, this kind of enterprise just likes to play this trick. Because the labor force is cheap, the job-hopping rate in this society is high. Why do they really want to give you a raise?

As long as there is no relevant salary description in the previous contract, it is not cheating, that is, you can't say goodbye to others.