They only run it back through the engine because they have to, it's the law.
I am not aware of any law, but they have been reusing auto engine oil since the 19th century. And though most of those years most did not even filter the oil! The microscopic metal is not a problem, not near as bad as the dust in the air.
Up until the early 70s all auto engines had a breather on the rocker arm covers venting into the great beyond. If they could get away from the smog systems today you wouldn't see it going back into the engine.
That is not the same thing, no oil other than a small amout of vapor went out that tube. The oil in the crankcase staysbehind and has lot of microscopic debris. Most but not all is filtered, but plenty of unfilterd oil is slung from the crankshaft flying around above the oil pan. Microscopic metal bits are not a big problem, most are very soft. The hard metals have very little wear in the first place. You get much more dust in the engine than the metal in the oil.