Because the rank goes to their head. It's just a letter, but they probably think it means they have the god-given right to act like an asshole to any players they beat.
I'm an A rank, and I hate it. Everyone thinks I'm one of those idiots. It's mostly just ADHD 12 year olds and slugs who live in their mom's basement, so they try to compensate for their lack of a real life by taking games waaaay too seriously.
Just sayin'