If the job listing states clearly to put, and if an applicant never put, it means an applicant does not even know how to follow simple instructions or is a maverick and a potential problematic person.
From HR perspective, it is to screen out applicants with expected salary beyond its budget and not waste both applicant's and interviewers' time. They are assuming that an applicant would not be willing to reconsider a figure lower than expected even if everything is right. Companies won't want to waste time interviewing someone they cannot afford.
Job applicants may complain stating salary lowers their bargaining power, it's true. Maybe a company is willing to pay even higher than one's expected salary if left unknown. However, the fact that one is applying for a job (and not being headhunted to fill a position), already means that the advantage is tilted to the hiring company/employer who is at the sole discretion to make a hiring decision. Put it bluntly, it is an applicant seeking a favour, a job or opportunity, and not the other way round.