Because they all got rifles/infantry probably. I just keep the cavs to kill intruders if anyone declares on me. At that stage I was researching for Tanks at 4 turns per tech so It wasn't long till my tanks were rolling through France, then I used the cavs to quell resistance in captured cities/metros. Right now i'm researching to upgrade to Modern Armour, still have 40 odd cavs which I shall use to rush the Mass transits my core are building.
Cavs are funny things to have in a tank invasion, I find them useful for scouting, since they are nice and fast, and taking out radar towers. And If you leave them sitting on their own the AI will use its own cavs to attack them, rather than going for any tanks you have sitting around. Which spares your better units.
Up till this point my games are fairly peaceful, wars for territory when I get upgrades, eg warriors to swords then to med inf, when I upgrade to Cavs is always a big one, then when tanks come along. When I get MA I am ready to totally overpower my remaining neighbours. My games are always like this; short sharp wars followed by periods of peace and consolidation. I like to get in the tech race. My ultimate goal is almost always Conquest though...
What I meant is - if you've got 60 extra Cavalry left over from some war, why not declare war on another nation to use some of them up? 
