The rationale behind academic and industry salary difference This question is not about PhD students, but about professionals with PhD degrees. To narrow down the scope even further, let my subject be a computer science PhD seeking a job in Europe. (I'd include the USA as well, but I'm afraid the dynamics are not the same. However, feel free to include relevant comparisons and data in the answers when needed.)

The salary of such a professional, if they stayed in academia, will be probably lower than if they chose to get a job in industry.

What is the economic incentive behind the university policy to not have higher, more competitive salaries when compared to industry?

I know that state institutions are (at least partially) on the government budget, so they arguably have little say in the matter. However, private institutions are significantly more independent in their organization. Basically, isn't it in the institutions' best interest to have the "best pick" of PhD graduates, instead of competing against the lure of a higher pay?
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!