Because when you are a EEOC compliant company AND are currently interviewing and hiring, you have a moral obligation to let the masses know that you are interviewing and hiring and give people a chance to put their resume in the hat, even if that hat is overfilled, and there MAY be no real current need for more resumes. It also keeps the pile moving (albeit very SLOWLY) because some of the resumes are underqualified, and you may get called 1-2 years from now.
Remember if they interview 20 people a month, and we have (for example only) 2000 "qualified" resumes in the recommended pile, it would take over 8 years to interview just THAT current group. We have more than that in the pile believe me.
As long as we are interviewing and hiring you will see advertisements letting people know and allowing them to submit resumes. This does not do anything to speed up that interview timeline as I described.
Welcome to the worst period of 121 employment that I'm aware of.