Wow this could have been a great discussion on a topic everyone takes for granted in this hobby. Instead it became an exercise in stupidity vs. ignorance. Troll on!
As it was already stated natural FW systems do have protein skimmers. I mean rivers, streams and creeks, not an aquarium when I type natural. I have seen on different southeastern systems skimmate being collected on the water's surface. Thinking about it each time there were several things in common:
1. Large nutrient source upsteam entering the system such as a farm or golf course.
2. There was a long riffle-run sequence.
3. There was a deep slow moving pool at the end of the sequence where the skimmate collected usually behind a boulder or fallen tree.
So you have a source, long contact/mixing time of bubbles and waste in the sequence, and a "collection cup" in the pool. Basically it is a very big skimmer.
So why not in a FW home system? Except for those people who spend the hundreds and thousands on a fish (lucky), I think it is mostly economics. Who is going to buy a $500 skimmer even if it works on $50 worth of fish? Yes there are some that would but overall water is very cheap to exchange (which it won't always be the case).