Over time the prongs can separate. Think of the custodian that sweeps the boarding areas and hallways in a particular area.. they're now 30 feet away from the plug, and have reached the end of the line as far as the cord is concerned and need to keep going forward.
YANK. Drag the cord to the next outlet. Wash rinse repeat.
When those outlets go bad, I can't see how any airport would want to replace them for "passenger convenience". Considering most airports were built long ago, the outlets were installed simply for custodial work and other work related to maintenance that requires power in some way.
Now that everyone has multiple computers, phones, etc and are all charging their various devices, all the time, you can't tell me that hasn't put a major strain on electricity costs. For this reason, I can see why airports wouldn't want to replace outlets. One less person that can charge their phone saves the airport money.