Brown envy can certainly exist, but I think there's a number of factors why some people might do something similar.
Personally me, my brows naturally do not grow thick. That isn't to say I'd remove them completely or anything...but I certainly do wish my brows were a little fuller. I think they'd look great. Sometimes it's just about creating a facial aesthetic that you're satisfied with whenever you look in the mirror
Doesn't mean we don't make bad choices sometimes, though...
*remembers her goth phase in high school and shudders*