You did write "how" instead of "why":) Anyway, giving feedback about whether a candidate is still under consideration or not is, simply put, the right thing to do, and I think that's reason enough.
Setting aside any moral arguments, it does benefit the company's bottom line too - hiring talented people is an important thing for companies, so it deserves to have some attention paid to the flow from beginning to end.
My operating theory is that companies fail to provide prompt feedback not for any other reason other than apathy or a lack of attention to creating a good process, and not from some strategic deliberation around keeping options open. So failing to provide the feedback is a sign that the company is not putting the proper attention to the overall hiring process.
Candidates can learn about hiring processes before applying, like with talking with friends or sites like glassdoor.
How the hiring process functions is a factor in where I choose to apply. For example Google has a bad rep on this - supposedly it frequently takes months for them to make a decision. As a result I'd never consider applying there.