Nobody is suggesting old jobs should be deleted from your database (I doubt any job listing website would do this). Just that the old jobs are too irrelevant to display by default as visitors are generally only interested in current vacancies.

Keeping the old jobs as a separate search option (eg "[ ] include historic items") de-clutters the primary purpose of the site. That matters when you consider that few people enjoy the process of job hunting so will quickly abandon any platform with a poor signal to noise ratio.