We are not sure yet, maybe Mbappe is just doing that to make his team spend more for him and increase his salary. Right now are see much news about PSG wanting to increase the salary of Mbappe and keep him in the team for the next season. That's why they did it the last time and it's not surprising to see the same story.
Mbappe is not doing this because he wants psg to increase his salary, his salary is already high enough and he is receiving huge bonuses from psg, he cannot ask for more from them, Mbappe is doing this because he wants to go to real madrid next season, but for free. Mbappe is a very good player, but i don't like how he behaves, how can the club invest all the money they have invested in him and he wants to leave them for free, that is not something psg will accept, that is why the club no longer cares who signs him, they just want him to leave, while they receive the fee for signing him.