Why schoolwork but not school work or school-work? Why is well-being the preferred form, not wellbeing or well being? Why help desk but not helpdesk or help-desk?
Occasionally I can suss out some sort of reasoning between the three types of compound nouns, like the hyphen in re-elect making it clear that it's not pronounced "real ection". But then you get silliness like "preempt" which obviously doesn't follow the add-a-hyphen rule.
I wish English compounds worked like those in German, and we could just string them all together into one word and be done with it.
ETA: And I think the re-elect and preempt examples don't even count, since they're not proper compound words but rather words with prefixes. So my understanding of the reasoning is back to 0.