Why does the youtube offline videos get expired
Apparently YouTube videos downloaded for offline viewing get expired after a certain amount of time. I haven't heard anything official about though.

The downloaded  videos are stored in the internal storage but they are saved as .exo file format which only the YouTube app can open (you can't play them with any other media player app)

The best advice I can give you is to use a YouTube downloader like Tube mate to download the videos.