Designers, like many people, tend to fall into habits and patterns. Their games fit into specific genres, so they tend to be similar to other games in that same genre. Best practice is to borrow systems that work well in other games. However, sometimes a system gets enshrined into the conventional wisdom; once that happens, they rarely turn a critical eye to these systems and see if they are really meeting the design requirements. MMORPGs experience this type of "inherited design" for many gameplay systems.