I was going to object, I've definitely heard songs in my dreams with fully comprehensible lyrics. I've woken up and immediately googled the lyrics and couldn't find the song at all. I found one with the same title that I had heard of but it was completely different.

The closest I've gotten to randomly coming up with music like that when awake is by singing songs in a different tune, but even then the tune was metrically the same. Came up with some bangers doing that, actually.