You make the classic mistake of assuming the combat units actually represent micro-versions of real military units.
They don't.
Like everything in a Grand Strategy Game of this type, everything you see is just an abstract representation of power-level.
Societies don't suddenly wake up one day and 'discover' things like horseback riding, it's just an abstract way of saying this tribe has now 'mastered' horseback riding & it implies that their power-level over a tribe without has increased.
When you see a unit on the screen, it's not 'a battalion of Horsemen', it's a visual representation of the local expansion of the military influence of the tribe that wields them.
This is why in Civ2, units had power-ratings more than just hit-points. Civ3 fails on both counts by having no power ratings, so disrupting the abstraction, but also fails to take in to account unit volume as a replacement, which, ironically, means that the only way to 'strategise' in this strategy game is to increase volume, but not because increased volume increases overall power-level, but because increased volume mitigates the poor rationale behind the combat system.
The whole point of having Horsemen is a reward for advancing your empire at a speedier rate than the opposition, they represent a significant power increase of everything. To then have that nullified by a tribe who has, almost literally, done eff-all for 1000s of years, and then say "well that's RNG folks! Ever heard of General Custer?" is missing the whole point by quite some country mile.
You are supposedly 1000s of years more advanced, have better cash-flow, better societal organisation, stronger culture, the whole shebang, and yet a tribe who's pretty much still in the early bronze-age can field not only just as many men as you, but have them be more than adequate at completely nullifying any great advantage you've made, you're not significantly better off fielding 300 shield's worth of Horsemen than you are fielding 300 shield's worth of Archers. You could even argue that you're worse off militarily as you'd only have 10 Horsemen rather than 15 Archers.
You could have a wonderful pedantic debate about what's better, 15 Archers or 10 Horsemen, but, in reality, there shouldn't even be a debate. The AI itself rates 15 archers as stronger in it's own programming as it just sees 15 x 2 versus 10 x 2. It's absurd from quite a few perspectives. The fact that you can get to Knights and/Medieval Infantry and still lose or red-line huge numbers of them when attacking a Bronze-age tribe, is the next level of absurdity which takes it into the 'obviously a bit wonky' territory rather than 'Lol random'.
That guy who said he likes it because it allows him to game the system is a completely different topic & anyone is free to like such a system, but liking the system doesn't mean it's not a technical train wreck.