I mean, let's be honest: Mbappé was there because he was getting paid a huge amount of money that nobody was willing to give him; Neymar probably at first was quite excited to be there because he wouldn't be in Messi's shadow anymore and he could have been the only real team star, but it didn't happen; then we have Messi who, just like Mbappé, managed to sign a ridiculous contract but he never cared about PSG or Paris, he didn't even learn a little French in 2 years... That's what you get when you have too many big names who don't care at all about the team but only about the money.
Well I think initially, Mbappe wasn't there for the money, he was there to play football and win trophies. The thing is, as humans when you get to an environment and you're the main man after working your way to be in that position, there will come a time when you earn certain privileges and thus when you see other people of your calibre coming in, you start seeing them as threats. Yes, initially Neymar left Barcelona because he wasn't given the due praises he deserved even in games where he made things happen. I could remember that comeback against PSG in 2017 I think, were Barca won 6 - 1. Neymar was the main player behind the successful comeback but that the end Messi received all the praises and I think that was what also angered him to wanting to leave Barcelona.