Why isn't aircraft structure inspection done by robots? Is there any known active research on the subject?
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.