How do you know they are? Just because a female dragon doesn't have the parts doesn't mean the dragons are all male. You waiting for a female dragon to swoop down and slap you in the back with her boobies?
It is an assumption I admit, however I am basing it on a few things.
1. None of the dragons with speaking roles speak with a feminine voice, and are all identified as male.
2. When a specific dragon mentions other dragons, they broadly refer to them all as their brothers in dragon tongue, despite there being a draconic word for sister as well.
3. There is nothing in-game that implies they breed, or are a form of natural animal at all.