What causes Messi's contract extension has not been renewed until now is a regulatory issue from FFP. Under the FFP rules, each club must have a healthy balance, or in other words, the club must not spend money outside the budget limit that has been set.
Messi has a very large salary, so with the arrival of new players in Barcelona (from purchases and the returning loan players), then Barcelona's spending will swell. To be able to hold Messi, Barcelona must sell some of its players or there must be more loans (to other clubs) so that Barcelona's expenses can be reduced.