Because it was written in the 80s and I guess theyāve learned from other times people tried to āupdateā theme tunes that actually no one really wants it.
Seem to recall thatās true of loads of theme tunes - they just write music you can sing the name of it too. There was a Radio 4 programme about this once where the guy played his first short at the Have I Got News For You theme and it was a sort of swing beat piece that you could, indeed, sing āHave I got news for youā over.
Still, no idea how come Game of Thrones ended up with a tune you can sing @xyloās name to.