It was rare to reuse graphics, even fonts. But music was a bit more reusable with credit of course. It also depended on your group, but most groups had an artist, not every group had a musician.
So those plundering the digital high seas would not only remove copy protection and write an "intro" to both boast about it and show their coding skills, they'd also steal music from commercial games.
And they were the OGs: the demoscene only came after the work (a bit shady of a work but a work nonetheless) of these pioneers.