Hoplites were the middle / upper class of the city - you had to buy / maintain the armour as part of your civic duties - you had a similar situation with the equites of Republican Rome. Since social equality was not a big concern in the ancient world you would expect the upper reaches of society to have 10, 100x the income of the average pleb and remember that arms & armour can be inherited rather than have to be replaced ever year or so.
Sparta took this to extremes where every Spartan was trained as an elite, and the rest of the peloppenese peninsula was effectively enslaved to provide for them
its only when you get to imperial rome that you started having armoured infantry as common place, and even that depended on whether you were in a full blown legion or just an auxillary
fast forward to medieval times and you had the elite knights, the peasant militias, and the yeoman / mercenary professional soldier who'd buy the best he could afford (studded leather hauberks from memory and my last trip to the royal armouries)