Living in center city Philadelphia, I can confidently say there are a couple unintended reasons these things may require less emptying: First, they're often so dirty that no one would want to grab the handle and open the door to throw trash in (I avoid them at all costs myself). Who wants to touch a trashcan even when it's clean? Second, if you're willing to grab the trash-encrusted handle and open the door, they're frequently jammed (or locked?) shut. On the few occasions where I was forced to use them because there were no other trashcans in sight, they were difficult or impossible to open.
So what do people do? They throw their trash on the ground, or they find a nearby trashcan that doesn't require contact to use. It's not unusual to see bags of trash sitting by these things with random garbage scattered around or on top of them.