But then you're likely to never even see them because there's no particular reason to kill enough things to garner any kind of appreciable chance at getting one. They have an absurdly low drop rate, like 0.000125% chance kind of low. I only got one because I got /absurdly/ lucky and found one within four kills on a new world.
According to the (almighty, infallible, omniscient) wiki, it's a 0.025% drop rate. So, roughly 1 in 5,000 enemies.
I've been attempting to farm for the jungle and crimson keys, as I got to see the awesome stuff in the hallow and frost chests (these keys both dropped on the same night, in a hallowed ice biome. I'd just gotten back from retrieving the hallow loot when the frost key dropped).
I even went so far as to set up a new world (hardmode, of course, since they don't drop before then) and sit on an artificial meteor biome in the jungle overnight in the hopes that something would drop it. I came back the next morning to find that no keys had dropped, but the RNG had managed to find a way of choking the number of meteor heads that actually spawned (there was at least one blood moon, and a nearby lake went berserk and pumped out so many fish that the meteor heads couldn't come anywhere near).
The problem with "finding them through normal play" is that you're probably not going to be sitting around any one particular biome long enough to actually have one of these things semi-reliably drop. Even the ankh and all its madness is easier to come by than the keys, and at least it has stages of progress where you can feel like you're actually doing something.
I think the new version took the random-drop mechanic a bit too far for a few too many things... But hey, I'm still more than impressed by the update and I'm having a great time playing, so I can't really complain. It's just that the keys are, really, quite aggravating...
...and since nobody seems to know for sure how they work, it's difficult to come up with farming stations that will actually do what they're supposed to.