Because you could get +25% with autocracy and still take urban planning. Not all your cities are building wonders either, and ancient units are so cheap that you might as well just spam them because there's nothing better to do. Even if you're not building military units, conscription is still useful. If you have like 3-4 archers, it's probably still better than most cards you have at that point. That means both autocracy and classical republic have one useless slot each (one red and one green). Also Veterancy is a good card. Take a serious look at your city yields when you first reach these governments. If your capital is making less than 10 of a yield, it's probably better than any amenities or housing CR will give. And how many great people points are you making?