Yeah, you're missing something... a criminal mindset.
Someone in the smash-n-grab biz isn't going to incur risk for what 'might' be there... it might be something of little value... or nothing at all. No, the opportunistic & enterprising thief goes after what he sees, and can figure out he can get rid of quickly, for cash.
The two big exceptions are drug addicts, who can get so desperate they become a total wild card (but they make too many mistakes and get caught quickly, for the most part), and people who knew what was there in the first place (so now you get to take stock of your 'friends'.)
There are a couple of other scenarios (like bumping your stereo through a sketchy area, parking, and then leaving the rig parked there a long time). But for the most part... 'out of sight, out of mind' applies.
Almost forgot... GM promoted two other uses for the 'hard' cover... one was as a parcel shelf. The other was as a preventative for having things roll around the cargo area (configured in the 'standing on end at an angle' position.) They even fashioned a couple of hooks into it, to use as bag holders.