Risks outweigh benefits for Business
There is plenty of evidence that states that a landfill site is very valuable, estimates are that an average pre-1980's landfill site could be worth as much as £500 million. But if you start mining a landfill, ANYTHING THAT COMES OUT IS NOW YOUR PROBLEM. If you find 100 tons of copper, that's great - if you find 100 tons of asbestos or cyanide (or worse!), that's an expensive problem. The keeping of records were poor at best for pre-1980 landfills, meaning there is no good record of what is in a site. Would you risk a large bill for opening up a landfill site? Most businesses and investors won't.
On top of this, anything that can't be recycled, or isn't already inert (soil, rocks, wood etc.) needs to be re-landfilled. This could incur an additional landfill tax which pushes the price up, especially if there is a lot of plastics, rubber and synthetic material.