For one, the destroyed ones have code.. or I'm sure they do, otherwise physics wont apply, nor will they dissapear after a while
Also, since each represents a vehicle that is probably in the process of spawning again, it takes resources that serve no purpose. Statics are factored into the map memory usage and so they can be hi res, while destroyed tanks need to be low res, so very different thing
Also, some statics CANNOT be used, even if they could be downgraded in pixel count, like the Stuart in the pic... I don't need to explain, I think.
Now why, the devs choose almost perfect black for their modules is anothe reason for debate - But in short, no. Statics cannot be used