I think it is because most people here make maps for RPGs, fantasy novels, or experimental purposes. In such cases, typically the important details are visuals (so it looks nice) and the specific locations involved in the game/story. The maps are designed for the adventures of an individual or small group. The tavern they visit needs to be on the map, as does the mountain range so you can have an idea of where they are going, but the borders of the world's kingdoms are insignificant. So borders are either not well defined, or not depicted.

Whereas when developing a map for the sake of developing a world, more attention will be given to borders because they play an important part in conflicts on the larger timescale.