They have evolved to keep their contents fresh and safe from the elements. Their shells are thick enough to serve as armor, but thin enough for a tiny chick to break through on its own. Eggs are marvels of engineering and do not need human tampering to be made beautiful. So why do people wash eggs? The answer is simple: It is because we were taught that behavior by grocers, factory farms, and people who are paranoid to get dirty. The truth is eggs are pretty awesome and washing them makes them less awesome.
As an egg exits a chicken, it is made wet with a membrane coating that dries and defends the porous shell from bacteria that may otherwise pass through and spoil the contents. Cleaning an egg with water removes that coating and reduces the egg’s ability to protect itself. This is why factory-farm eggs need to be refrigerated. A farm fresh egg (one that has simply been wiped clean) may sit unrefrigerated, just as it was designed to do.
If an egg is perfectly clean it is likely from a factory-farm and washed to look pretty. Or perhaps it is from a farmer covering the fact they do not clean their nest boxes nearly enough. Either way it is likely the hen who laid the egg is not living a pleasant life. So next time you buy eggs, look for the ones wiped clean and with perhaps a little poop, because that is okay. It wipes right off.