The money they've offered Messi can convince him to leave Inter Miami for SPL but he doesn't want to play in a league which Cristiano Ronaldo's the main player. Messi's the main player in MLS & Cristiano's the main player in SPL he doesn't want it to change.
I don't think that that is the reason and money is not everything. Messi's priority is to settle in a good environment with his family. Which is a better option, the United States or Saudi Arabia for rich people? It's hard to answer this question because everyone has different priorities. I would live in the USA because it's very rich, you can go in any state you wish and Miami has also been my childhood favorite place (Do you remember good old GTA Vice City?). I think that Messi prefers Miami for himself and for his family. I remember that he said this in one of his interview soon after winning World Cup.