Vertical speeds are more difficult to predict and control by the computers and controllers.

Bingo! I think we finally solved the mystery as to why the hell ATC seems to be so inclined to drag airliners in low. The problem boils down to the fact that during descent, an airplane is inherently occupying a much greater volume of airspace then they are in level flight. Far out, there is plenty of room to accommodate that uncertainty volume that the airplane needs while descending. As you get closer to the terminal, there is less volume to work with (1/r^2) and the controllers are wanting tighter control which they can get with level flight and smallish altitude changes.

I don't like it one bit, but that's undoubtedly why it's happening. ATC needs to recalibrate the risk that they're taking by running heavies through airspace shared with gliders. As others have pointed out, they're likely oblivious to how prevalent gliders are and how hard we are to see and avoid.