1. It has to do with cash
2. You might tell this to a kid
3. Nature is somewhere in this proverb

You would use this expression maybe when a child is asking for too many toys or candy and you want to explain to them that money is somewhat precious, so you can't be buying stuff all the time, so you tell them 'Money doesn't grow on trees.' It simply means that money is simply not common, and has more value than the child really thinks.