Because they promised DNCG (entity that monitors French clubs' accounts and can punish them if they screw up) that they'd sell players for minimum 100 mil between last summer and the winter transfer window. But so far they've only managed to sell Bakker to Leverkusen for 7 mil and get pocket change for some younguns who didn't get game time. Cause Leonardo is an even shitter DOF than Fabio and who would buy Paredes, Draxler, Kurzawa, Icardi & co knowing they're gash and/or always injured but make zillion each month in wages. So PSG are fucked and can't get rid of them. Result is, they're not allowed to add to the squad unless they get rid of their expensive unwanted first.
Personally, I think there was a deal to explore where they'd have gotten N'Dumb and we'd have received a bundle incl Gini, Kehrer and a couple of interesting youngsters...like Xavi Simons or Bitshiabu now and Kalimuendo later when he comes back from his loan at Lens.
PSG would've got Mbappe's best mate and it may have helped convince him to sign an extension. Also they'd have cleared the decks and saved a bit of cash.
We'd have gotten a proven PL star in Gini and that CM creativity we desperately lack, a guy in Kehrer who is still young and who could become real reliable under Conte, and a couple exciting youngsters who could turn into bona-fide stars in 2 or 3 years...and if not, not much harm done.
So yeah not sure how it all came to nothing when there was something for both teams to gain from striking a deal. Ah I get it...you had Fabio on one side and Leo on the other. That explains it