Eg "the color is wrong because we specified it wrong" and "the color is wrong because the app doesn't respect what we ask it to display" both ends up with the same bug (wrong color), same priority, but not the same severity because the second case is uncontrolled.
Severity is a dev tool, priority is a business tool.
1. We have the wrong .png asset in the database
2. Our entire rendering infrastructure is suspect
I'm not saying this is the wrong approach by the way, it's just interesting how people approach this differently.
Of course it does add complexity. It is the call of each organization which things are important enough to be worth the extra complexity and which are not. Maybe for yours it isn't worth the extra cost - there is nothing wrong with that - but other people have different needs and so they will have different answers.
In short, you are wrong in thinking there is a universal right answer.