It's a closed system isn't it? Where does all dirt and sludge that makes motor oil "dirty" come from?
Those black particles come from two processes.
The oil burns a little along the cylinder walls during combustion, and this is scraped off into the crankcase by the movement of the pistons. There's also some non-combustive oxidation of the oil just because it's in a hot, kinetic, oxygen rich environment.
The second component of the black crud in your engine oil comes from tiny bits of metal that have worn off of the moving surfaces during engine use.
The buildup of these impurities in the oil leads to increased wear, so the oil needs to be replaced from time to time.
It's kind of like the way you can wash a certain number of dishes in a single bucket of water before the water is so filthy that it can't be used and has to be changed.
