^they most certainly did, it's just that for a decent amount of time the Spartans were the only full-time professional warriors (a warrior aristocracy more accurately) in the context of pretty literal Sunday soldiers. Once Greek warfare got more organised, professionalised, combined-arms and generally just increasingly more complex than "two lines of hoplites charging each other on level field" they quickly started floundering pretty hard and the fucked-up domestic organisation that had allowed the establishment and maintenance of the whole aforementioned warrior-aristocracy thing in the first place started biting them in the ass.
One of the more clear-cut cases of the Crippling Overspecialization trope in real-world history that lot.