I think it is more than him just not being interested. He needs a team around him willing to run for him and be set up specifically to get the best out of him. That is not a problem as it was a system that took Argentina to the World Cup. As much as the narrative suggests Messi carried a team of average players to the World Cup, it looked to me like a team doing the (really) heavy lifting for its star player. Kind of like a tired mule having to carry a fat-rich businessman up a mountain so they can say they reached the summit. It is not even a criticism, it was clear Argentina could only win the competition with Messi doing an impression of his best, so they found the methods to allow that to happen. He was fantastic in the World Cup.
The problem is at PSG it isn't happening, and it is not happening at any top club in Europe. But specifically at PSG playing to suit Messi wouldn't work. Firstly, there are other star players who will not become grunts for Messi to shine. Secondly, the players in the team are not the kind of "work hard, passion, and grind" sort of players. So, it seems to me Galtier tries to make Messi useful in another way, by making him the passing link between the lines in midfield and attack. This works in Ligue 1 mostly but against a well-organized top team, it just becomes easy to take him out of the game.
It is what it is, he is 35 now and is still an outstanding player. I want to see if there is a coach who can take him (for PSG or another team) and find a way for him to be useful in big games.