First of all, prefabs are segments only which means walls/stairs/doors/windows, you'd still have to put lights and furniture in yourself.
Maps are the complete ones, including everything.
The catch is, that other people can only use prefabs if every single segment, mesh and texture is in exactly the same folder as with the one who created the prefabs. This is rarely the case and for maps including entities as well it's even worse. It would work with prefabs made from single Model Packs but most people mix MP, stock media and free stuff in their maps.
Besides I think that level design is THE most fun part of FPSC, I wouldn't miss it
In case you find my grammar and spelling weird ---> native German speaker ^^