This is because we assume that the highest level of football is in Europe, and that the world's top five leagues are located there. We still assume that the pinnacle of a footballer's career is playing in Europe, for a big club in one of the top five leagues. Meanwhile, leagues outside Europe don't offer adequate competition. Domestic leagues like the Saudi Pro League are quite competitive, while other leagues on the same continent are less so, thus the continental competition is also less competitive. It will take a long time to create a competitive enough competition to make it one of the best leagues in the world.
Currently, the branding and popularity of the Saudi Pro League still depend on the signing of star players, and when those star players leave, the SPL will return to being a league with little interest.
Assume? The league in itself could be competitive, like we have seen different winners so far, so I understand there is a competition in Saudi league between each other. However, league by league, there is no way that Saudi could compete with European leagues.
And by European leagues I mean the top five, because if you really want to compare Slovenian league to Saudi league, of course Saudi league is better, and if you really want to you could even compare the bad teams of top five league to top Saudi teams, that also works and Saudis would win. But top teams of top five? Would easily destroy Saudi teams in a league structure. City did lose, I know that, but City has been terrible for a while now, it is not a good example at all.