A hypothetical example is that a small toy maker comes up with the idea for the Big Wheel (https://www.amazon.com/Original-Big-Wheel-Inch-Tricycle/dp/B...). They patent the design for the concept and begin manufacturing the Big Wheel. Once on the market, a big toy manufacturer could easily begin producing the exact same product because they have the resources to, so the fact that it is easy to reimplement does not mean that it is contradictory for the inventor to want a patent. The patent on the 'invention' of the Big Wheel would be the only thing the small inventor has to protect themselves.

I know that the world and community has benefited from open source and freely available software and that there is a bad history of software companies using the patent system in debatable ways, but there are also cases where it can look like big companies take advantage of the free and open ecosystem in ways that disincentivise future innovations.