Well Messi is probobly full of playing football and just wants to have fun a little and earn a lot more money in the last years of his football career. After he won the world cup he dosent have to win anything so there isnt anymore motivation for him to play at the top quality level as before.
It is pretty obvious, that all those transfers that are happening in the last 2 years or so of very big and very famous players to clubs from leagues that are absolutely irrelevant in the world of football are only happening because those players can earn a ridiculous amount of money there. I mean those players have earned between 15-25 Million € per year before too at least, but now at those clubs they can get ten times of that per season which is just crazy.
A single one of those players is earning more money there in one season of playing for a club that is absolutely irrelevant than a whole village of engineers and doctors will earn in their whole life.