Well, some phrases (and yes, phrasal verbs) get concatenated over time, like "understand", "overtake", "courtmarshal", "becalm", "whipsaw", etc. The conjugation and number rules work themselves out eventually, for example, "courts-marshal", "overtook".
It's just one of the ways languages mutate. Usually the noun form mutates first, and later on the verb form, but not always.
