Das Madchen = Girl, but Boy = ?
Why is " girl" so commonly used in the lessons but not "boys"?
Just an observation!
I'm not sure whether this is done intentionally or not. But if so, the two main reasons I see (and already mentioned by Lynoure) are:
It's good for English native speakers to be exposed to 'das Mädchen' a lot since it's very counter-intuitive for them to use the neuter gender for girls
'Junge' follows weak declension and not the more common strong declension. It can be confusing for learners if they are exposed to both schemes too early.