Because of the evolution of language and courting and how these phrases were adopted into common use in 1900s (1960-ish is when they were popularized). Prior to that people would call their romantic partners their beloved, intended, or other terms that denoted endearment or relationship status. Dating wasn't really a thing until the a bit later into the 1900s anyways, and instead courtships were commonplace up until the turn of the 20th century (and even then it wasn't legal to "date" until some years into the 20th century). In the 1960s when the terms boyfriend/girlfriend were popularized through popular media, people were also using terms like "my steady" or "my old lady/man". Through the shift of time "girlfriend" and "boyfriend" stuck around while other terms fell out of popularity.
People have been shifting to more gender-neutral terms within the last few decades, though. "Partner", "lover", "bae", "hubby/wifey" are in the upswing, and I imagine in the next 20 so or years, the gender-neutral term will likely become the more common term over "girlfriend" or "boyfriend" just because of how more conscious people are about inclusiveness.
I know it seems like I'm giving you grief about something so silly but you literally said you would call Mickey's VA a guy but call Alyson Stoner--the voice actor of your favorite character at that--a girl and it shows you aren't really seeing where the double-standard is. You say the terms are interchangable in your culture but there is absolutely no way that you'd say that if say...Sora's VA was replaced. You'd say "new guy" like you would Mickey's VA because it is the common colloquial phrase. It still denotes respect to that person. "New girl" has no respect in it, least of all to somebody who again has been around for 12 years. New or girl is the last thing Alyson is.