The other thing with the weight of the OE hard cover (for me anyway) is that it makes it a bit tricky to open. You have to pop the rubber cover, insert the key and twist if while at the same time lifting the cover with one hand. Then you can remove the key and I like to put the rubber cover back on, as it acts as a bumper with the cover folded over onto the other half and I don't want to take the chance of damaging or loosing one of those ridiculously expensive lock covers. If you're opening the rear half, it's twice as hard to do if the tail gate is open, because you have to stretch to lift the cover up, The hard cover is very strong. I'm pretty sure I could easily stand on it (250 pounds) without hurting it, which accounts for it's weight.
BTW, The lock retainers are prone to falling off and getting lost. I had one missing and the other was very loose, I bought new ones (OE Ford) and found they fit very loosely too. I ended up bending the spring steel a bit more to achieve a tight fit,