Isn't the basic problem that instead of being buried and forgotten - so that they are found a few generations down the line, when their contents will be unusual and represent thngs that then belong to a bygone society and culture - they are being actively sought out within living memory by folk interested in getting little more than a headline, and a little publicity?
What could be a valuable future resource, as has been demonstrated in the recent 'Glasgow clippings' thread here, is being destroyed by people who have no real interest in history, just their own importance.
As Ronnie noted, there's nothing exciting about a 10, 20 or 30 year old capsule being opened, most folk probably have better stuff lying around the family home, but put another 0 on the end, and that's a different story.
I blame Blue Peter. As I recall, they were forever burying the things, and digging them upa few years later. Great for kids who probably weren't born first time round, but it's done real damage to a potential resource.