Mario Kart - Rivals

I find it funny that Mario's "rivals" is both Peach and Bowser.
Like he just found out last minute before the race that Peach chose Bowser over Mario because of Bo[Stockholm Syndrome]k .
Now, Mario has revenge to beat his Ex and the overgrown homewrecker.

It's strange that King Boo isn't considered Luigi's rival. Like I can get Mario (brotherly rivalry), but you'd think the main antagonist of Luigi's mansion would be the rival of the protagonist.