English nouns have a person/nonperson gender distinction that requires agreement by relativizers.
Compare:
1. The girl who I met yesterday
2. *The burger who I ate yesterday
This gender distinction is hazily applied to the third-person pronouns he (person), she (person), and it (nonperson), which is why different people may see referring to babies with it as normal usage, or as grossly offensive.
But in German the girl you met yesterday is neuter, and the burger you ate yesterday is masculine, and you need to know that to form a grammatically correct sentence about them.
You failed to distinguish the concepts even in your own terms; a girl is a person, and a burger isn't, and you need to know that to form a grammatical sentence about them.
Even though a girl is a person the gender of a girl (das Mädchen) is neuter in German. You need to know that grammatical gender to form a grammatically correct sentence - you do in fact need to say "it" ("er") in your sentence referring to the girl you met yesterday.
A girl is person and a burger isn't, but in German a girl is an "it" and a burger is a "he".
Taking sex-based gender distinctions for our example, we can observe that Chinese speakers aren't able to correctly choose between the English pronouns he and she, despite the fact that this distinction isn't arbitrary in the slightest (and Chinese speakers routinely make it themselves in other contexts!). The reason is simply that their native language doesn't make the distinction, not that the distinction is theoretically difficult to make.