We are working on adding the filter you are requesting. This filter would show you job postings that are new to you.
However, to answer your question, some hostels prefer to keep their hostel job listing posted all the time because they continually have opportunities at their hostel. Whenever a hostel updates their listing it rises to the top of the search. If they want the listing removed they can do this also.
In the past, we removed a bunch of listings that were old and a lot of hostel owners complained stating that they wanted their job posting to stay live all the time. So now we leave it up to the hostel to decide when they want to remove the listing.