That’s because it ends on “-chen”. It’s a suffix to basically cutiefy whatever comes in front of it and makes every word agender. So like “Der Hund” (the dog) becomes “Das Hündchen” (the doggy). “Mädchen” originally comes from the word “Die Maid”, which means young woman but is never used in modern German. So “Mädchen” is basically “girlie”, not “girl”. Don’t ask me why this happened