Are hiring ads just a marketing gig for the company?

I've been keeping track and noticing hiring ads for a while now and have been sending my resume to quite a few companies lately. Some of them get back to me but there are many who never bother to reply. And these ads are lingering around for months. If they are not going to hire why advertise.

It all makes to think they are merely doing this for the marketing purpose of their company, product and to let everyone know they are working on cool stuff. Seems quite pathetic and this trend is prevalent these days.

What do you guys think ?