Tenants are normally unhappy after their move out if they don’t get all of their security deposit returned. It is very rare that a rental is returned in absolutely rent-ready condition (less a little normal wear and tear). Moving tenants are pretty tired after all that activity and cleaning the place spotless becomes less of a priority. It’s human nature and it’s understandable.
Just imagine if car rental companies charged you for not washing the rental car upon return. Why don’t they? Because they don’t take a security deposit to make sure it is done, because you bring it to the location where it will be washed (cheap) and because they build that service into their pricing model to begin with anyway.