What benefit is there to employers to have people languishing with green cards for years on end? H1B -> permanent resident can be a long process, but it is rar

What benefit is there to employers to have people languishing with green cards for years on end? H1B -> permanent resident can be a long process, but it is rarely the longest part of the process. Once they reach the permanent resident stage they are more or less equivalent to a citizen with respect to employment.