Speaking for the snowy fence I once created: I just wanted a snowy fence. Recoloring the existing one was indeed least time consuming.
Another reason is probably the so called "rotation fixing" which had to happen with non-recolored fences. With all these different sides and angles (lots more than for a building or even an animal), that would take up a lot more time too. Especially in the early years, proper tools were scarce to do it in a fast way.