Because other teams pay them more and have a bigger international pull. It has nothing to do with the manager.
That's one argument, sure. I can't really discredit that in any way. We clearly have the money to keep them if we wanted to though, if we can pay 90m for Higuain and 30m for Pjanic, we can bump Pogba and Vidal's salaries by a few million euros if it means we keep a world-class player from leaving us. If all they wanted was a pay raise and they'd be perfectly happy to stay here if they get it, it would be foolish to let them walk.
My argument, and this is backed by an actual statement from one of my two examples (Vidal) is that they leave because they think they have better chances of winning the CL in their new clubs. The team that reached the final a few years ago, in terms of players, is a world class team that could have had a period of European domination for years, like Guradiola's Barcelona did. World class players would have flocked TO that team, not away from it, if we had a name like Ancelotti (random example) on the bench.
Dybala leaving would make it three world-class players in three years, and alarmingly, two world-class young players in two years. Losing one of them is a tragedy, losing both is essentially saying "Meh, I don't care about this whole 'future' thing."