Starting from the salary and the level of comfort, because the Saudi Arabian club is ready with everything. This will continue and top players are starting to think again about their services at European clubs which are often underappreciated.
Eh?
I never knew that most of the European league players do not feel appreciated by their clubs enough

Seriously though, most players are actually quite happy to stay where they are. The Saudi Pro League can't simply buy everyone inside all the leagues (or even all of the best players in the leagues), there's not enough room for that, and its quickly unsustainable even with oil money, without a doubt capable of causing insolvency of the buying clubs.