For better or worse, it’s actually a very common practice in songwriting. The riff, however crucial to the song it is, was merely part of the arrangement. In a band setting the person who mapped out the basic song structure is the one getting the songwriting credit.

For instance, Freddie Mercury is credited as the sole writer of Bohemian Rhapsody even though the legendary guitar solo was entirely composed by Brian May.

It could have been the artist’s intention. Manic Street Preachers’ The Holy Bible is that album to me - cold, dry, dark. It’s meant to make you uncomfortable, and it was done masterfully

Sometimes I Make You Sad by Supergrass.