>because the software infrastructure for measuring that doesn't exist (though I suppose someone could write it);
What the hell are you talking about? The endermen in minecraft change behavior based on whether you are looking at them or not, and minecraft has hardly a bastion of bleeding edge game engine technology. The difference between making an enderman attack if you look at it for more then 2 seconds and firing a metric if you look at a coke bottle for more than two seconds is pretty minimal.
A metric tool that sends data on how long you keep certain objects within the center of your FOV can be backed with off the shelf free software[1]. The infrastructure is commonplace, well tested and used daily for a myriad of metric related tasks all over the world.