Acording with what said in the description of pokedex and explained in the anime some times, some Pokemon use man mande objects when they can get or are traveling with humans, however wild ones use another things in place of thoose objects, Throh is a good example, here is what the description of his pokedex says in Pokemon Black: "When it tightens its belt, it becomes stronger. Wild Throh use vines to weave their own belts." You can find that description in this own website.
But their in-game image are only the standarth, the reason are uknow, but maybe is just because making a game require alot of work, and creating too many sprites and animations cold slow the work and their projects and don't have an huge impact direct in the game or his mechanics. Mostly a development thing.
