Why isn't aircraft structure inspection done by robots? Is there any known active research on the subject? From my research I've found that (correct me if i am wrong):

Structure inspection (in B,C and D Checks) including (visual, thermography, ultrasonic inspections) looking for structure damage of different type of composit materials (like cracks, dis-bonding, delamination, water ingestion, holes, dents ... etc) in commercial airliners specially wide body aircraft takes alot of time, labor and cost. 

So why isn't it done by robots which should be more time efficient, accurate and reliable? in a way that robots can do simple but mandatory inspection tasks allowing operators to focus on more skilled tasks (like engine work) reducing the overall plane down time !

Is there any robotic inspection systems exists that I didn't hear about? 

How beneficial to airlines would it be to have a robotic system that would assist operators in the inspection process (in terms of time and cost) ?
The robot, or better the software analyzing the sensor output, needs to be programmed for every eventuality. After all, wear and tear does not leave standard traces, and a human is still better in spotting when something does not look quite right. This does not concern the obvious cases, but those where experience and intuition make the difference.

Next, aircraft structures are made with human inspectors in mind. A robotic system would need to be able to access all points at least as well as a human can. This puts some heavy restrictions on the robot's physique.

Eventually, a robotic system will surpass most humans, but only after thousands of man-hours of training. Maybe someone is developing just such a system right now, but I am unaware of this.

One point remains, however: Who will be liable if the robotic system makes a mistake? The manufacturer will need to buy insurance to cover those corner cases where the robot fails, and this will come on top of the hefty expense for developing and training such a system. I expect that the military (e.g. DARPA) will be the first to make the plunge, and only when the robot has a positive track record will civilian operators start to consider using it.

The EasyJet drones are just helping to get a camera on top of the aircraft; the eventual monitoring of the feed will still be performed by humans. They are a high-tech version of a periscope, if you want.