That's a very good question Rosie!
I have run into it quite often myself and wondered why the creator didn't do more to identify the seperate meshes with distinct names, especially when they had to create them and then group them for their own use in creating the models(s). It's also frustrating when they group non-related meshes together that have nothing to do with each other like say, right engine nozzle and wings together, and then left engine nozzle and fuselage together on an airplane model. That makes it impossible to apply the same materials to the engines or seperate materials to the other parts.
I really don't understand why some go to the trouble of creating seemingly complicated models and they turn out to be one single mesh that has no seperate parts at all to them or why some complicated models are so sparing in their wireframes - you know those that have wireframes that seem to disappear as though they aren't connected to any other part of the model, buildings and terrain meshes being prime examples!

I know the answer will probably be that the creator did it for his/her convenience in creating the mesh and not the end user's, or that I could create something myself if I had a felt it was so easy to do. Unfortunately, I can't model and that's why I spend money to buy their models or download free items. But I still wish the creator could do better about distinct mesh naming and group seperation/integration sometimes - especially so they could facilitate the end user in applying thier own material variations to correspondingl meshes or specific parts of the models.