When they usually aren't all that classically attractive themselves.
So stupid. And it makes it seem like they worship attractive people.
Bella ramsey would be an example in which a bunch of chud nerds call her ugly constantly meanwhile they're not exactly lookers themselves.
Complete speculation:
The “attractive privilege” is , usually at least, one of the few they don’t have access to. They are usually white, they are men, they tend to come from the imperial core.
It’s like the one type of privilege they don’t get to experience, setting aside that working on themselves is an option, and for that they’ll never forgive society. They lash out and seek to tear down everyone they perceive as “unfairly” benefiting from the genetic lottery (with a complete lack of introspection into all the other lotteries they rolled better on).
And then there’s all the well-trod ground of typical incel bs that this is just a part of. It’s like they created a self-sustaining ecosystem of rage, bitterness, and self-loathing.
Edit: Dirt Owl is probably more on the money for this, it gets straight to the core of it.