Why are superhero books so unpopular?

This isn’t a complaint or anything, just an observation I’ve had and I’m legitimately curious. You’d think in an era where supehero media is at its peak popularity, superhero novels would be more popular, and yet I never really hear about them the same way I do fantasy, mystery/thriller, or romance. Is it because comic books already exist so traditional literature feels redundant? Is there just not a market for superhero stories the same way there are other genres?

Has anyone else noticed this, or am I just being ignorant and supehero books are actually super (no pun intended) popular?