They pay Ronaldo well because he's not only a good player, but also a global promoter of the league.
They prefer this league because it offers good pay for established star players. Perhaps in 3-5 years, younger players will join the league as well.
The Arabs are very wealthy due to their oil resources, and if they continue like this, they will make this league one of the leading leagues in Europe.
I think Al Hilal also did quite well in the FIFA Club World Cup.
^^^ This
Ronaldo's value is not in the number of goals he score for Al Nassr, or the games that he win for them. His real value is in popularizing this league. Before Ronaldo joined Al Nassr in 2023, Saudi Professional League was a relatively unknown competition, with hardly any viewers outside Saudi Arabia. But now it has become one of the most popular football leagues. Also, Ronaldo was on the forefront during the Saudi Arabian bid to host the 2034 FIFA World Cup (which was successful).