Having an army is expensive.
“expense of doing this would have been in most cases enormous and of course those are resources that could be deployed to any number of other uses, including simply raising a larger army of less disciplined, well trained troops. For, say, a Greek polis given the choice between having 450 men under arms all year round and excellently trained or having 5,000 hoplites under arms only in the one month where there is a battle to fight the clear answer is the latter. A lot of things need to line up before producing armies with high levels of synchronized discipline makes sense: the state needs a lot of resources, year-round security challenges and a tactical system which is effective and demands this kind of investment. So while developing synchronized discipline offered an army a degree of greater control and flexibility, it was prohibitively expensive.”