Because terms are rarely as objective as we like them to be. A term or concept only makes sense if you know the lingual networks around them so to speak. If you take a concept and use it to describe something outside of that network, say a different culture or time period, then it can never work on its own. It only works for people who understand the network you're working from.
And this is problematic for several reasons, one of which is that you're forcing the people you study to adopt your language of explanation in order to enter the conversation. Not as relevant for history, because ancient people are dead and can't speak anyway, but the same problem is structurally visible in how hegemonic academics talk about minorities for instance.
So basically you give someone or something a label so you understand them, but the language you use is your own product, so the only party that is in the position to accurately confirm or deny the labelling are the subjects themselves. And historical figures can't do that because they're dead. Living people can do that, but it's still a bit of a dick move to force people to afopt your own language in order to affirm your own concepts, yet unfortunately that's how the world works at the moment.
I hope my wall of text was helpfull, this shit's vagua as hell I agree.