That's my experience too:
Swarm tactics are almost always an
AI strategy - especially at the Deity/Immortal level, but not uncommon (iirc) at the easier levels either. The AI's ability to outproduce and outpromote humans makes trying to compete against them on numbers an unattractive early strategy most games. So the inexpensive warrior swarm is attractive for the AI, not so much for the human.
In a human vs. human competition, the warrior swarm has its attractions.
If one can carry it out, of course. Most players aren't indifferent to a fellow human building up a bunch of warriors - 'cause unlike the AI, the human is going to use them and not simply pay the bankrupting
maintenance costs for nothing. So if you see your human neighbor gearing up, then you're likely to do the same. If you can't match 'em in numbers, well then you're in trouble anyway because he's likely to have other advantages like more cities and resources.
The
Iron Working tech, to get back to the original post, is expensive and I understand how once one has paid the thousands of

, one might expect the military to be greatly invigorated but instead finds that it's an advantage but not a game changer: wars go on as before. However, this is something all the techs share: you spend beaucoup beakers researching some school of magic and your enemies still survive. I don't think it's because of an overcostly tech or an underpowered advantage.
The real culprit is the lowly warrior: he happens to be the best "bang for the buck". He's a beast, a brute, the scum of the taverns and docks, the ignorant peasant from the fields - but he's the "Saviour of the nation" when his people need him. You don't have to pay much to train or equip him - just give him a pointy stick and point him in the right direction.
He makes other units look too costly. It's his fault.