Why didn't the follower limit scale with level?

With D&D war gaming background, it makes sense why early games emphasized the leadership element of CHA and how many followers you could have. It fits with the explicit progression of characters from low level nobodies to lords and ladies.

Why, then, does the number of followers you can have depend entirely on CHA and not also on level?

I'm thinking specifically of the videogame Mount & Blade, which uses both. I think it would fit an OSR game even better, where anyone can become a lord with a large retinue, but early on it really helps to be charismatic.

I get that at thise high levels you aren't literally leading armies determined by your charisma score, but it would make sense to me if Charisma and level (probably class, too, favoring Fighters) scaled in such a way that at the very high levels, you were literally leading armies with a number cap based on that score.

That could definitely be unrealistic to just have an arbitrary limit. Maybe it could be more of a "lead well" scenario, where leading an army X% larger than your capability imposes morale penalties.