On the contrary, I don't think that pilot episodes are often unaired. They are usually aired as a show's first episode unless there are issues with the production quality, the cast, or similar.
From Wikipedia:
Most pilots are never publicly screened if they fail to sell a series. If a series eventuates, pilots are usually – but not always – broadcast as the introductory episode of the series.
A pilot episode is generally the first episode of a new show, shown to the heads of the studio to whom it is marketed.
From TVTropes:
Even when a show is picked up and given a timeslot, there is no guarantee that a pilot will ever reach the air. They often do, usually as the premiere.