Because you can't conquer many cities with just couple of full stacks. You siege a city, and either wait for it to surrender or take on the garrison. Then you set up the temporary admin... and realize that the place is full of strange people speaking gibberish. And they are NOT happy about being your newly made subjects. You can see the next city a turn or two's march away, but to take it you'd need to remove your army from the newly conquered settlement. Which will make the locals kick out your governor and magically zap up a bunch of either nasty elites (if you didn't trash their MIC) or nasty triple-gold-chevroned levies (if you did trash their MIC). And what's much worse, the settlement didn't went to the Eleutheroi, it went back to the faction you just took it from
In other words, you'd need to build up a horde of garrison troops to actually KEEP the provinces you capture. Which is really too expensive to do, unless you're swimming in money for some reason. It takes a long time of constructing infrastructure in the captured settlements before you can keep peace with a reasonably small garrison. By that time, the AI faction will have spammed you with several more big armies.
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