I can easily imagine it.

1) There are plenty of people (complained about on FT constantly) who drop their carry-on in the first available overhead when they board, then move to their seat, which might be much farther back in the cabin than where they dumped their bags. So, some people don't have to be "forced" to separate themselves from their bags.

2) Certain bags are far more likely to contain valuable items than others. It's readily apparent in many cases from the bag's brand that it's a camera bag, a laptop bag, or a camera/laptop combo bag. Not only that, but certain brands of luggage are quite valuable, regardless of their contents. And of course, let's not forget that many folks will open their bags, showing the contentst to all around them, either at the gate or in flight. So, no randomness is needed; a thief can see potentially lucrative targets and limit his operations to those most likely to net him a profit.

3) You're right, thieves are not generally the brightest bulbs in the box. However, there is an overabundance of chronic stupidity in the human race, so no matter how dumb a thief might be, there are always dumber people around who make inviting and easy targets for them.