The rationale behind academic and industry salary difference
I highly doubt university salary policies are based on "economic incentive", especially in European countries where universities are directly funded by the ministry of education. Generally, political, not economic reasons prevail.

That aside, competition for academic positions is already fierce - there is simply no need to offer higher salaries to attract candidates. Currently, candidates are attracted by reputation, the thirst for knowledge, and similar non-monetary considerations. It is by no means clear that higher salaries would attract better candidates: you would have to argue that someone who simply follows the money would make a better researcher than someone who is willing to put in long hours at low pay for the dream of someday being allowed to put "Prof. Dr." in front of their name!