They pick a plane that did very well, that looks really good and upload it as their own, but the plane they upload is easy to recognize usually, as it is in the highest rated, was featured, etc. they upload it in hopes that people will think they worked hard, but it just makes them easier to catch. If they copy an insignificant plane, they know it will not get upvotes, so they take a massive risk and upload a popular plane in hopes of taking in the upvotes, fully knowing that up to several thousand people could expose it as a copy, even if they use the subassembly method