My personal opinion is that the European laws and immigration regulations imposed on universities are very restrictive, in a sense. This is not the question of money only to attract outside-EU scientists—it is a question of prospects and social stability that are offered not only to the candidate for a specific job, but also to his/her family.
My impression is that these restrictions are mainly related to the fact that the university bodies in Europe are requested to prove to immigration authorities first that no EU-citizen exists to compete for the advertised academic position and just, thereafter, the university bodies could hire the candidate that is not an EU-citizen. In other words, they need to prove not that the candidate from outside EU is the right one they want for the job, but that there is no better one home-raised. It is somewhat ridiculous since no suitable candidate from within the EU might have been interested in the job, but there could be another suitable candidate from outside EU.
The fact is that scientists from all over the world are directed to the United States, mainly. And, unless this EU rule is changed, many outside-EU researchers will buy one-way tickets to the United States.