The develop graphics for the game. Hence, developers.

Although in the context Graal users normally use, developer refers to a coder/scripter.

And yes, Graphic staff could work on other things besides hats. But there is little room for them to create stuff in the whole scheme of things BESIDES hats. It is hard for a group of graphic staff, especially an individual, to throw together a special update. Anything that isn't hardwired into the system such as the hat system or the mount or morph system is a hard project for hobbyist pixel artists to throw together.