Why won’t my downloaded songs play with no service?

I have all my music downloaded onto my phones main store (it takes up like 55 GB). But if I try to play songs with no service it won’t let me & just stops. Why? (When I play the song it just stops it and won’t continue)

I noticed this on a plane ride today & it makes no sense.

I also went in a couple other plane rides this year and didn’t have this issue. So I was surprised to see this