Because they usually don't buy too expensive players and don't pay them too high salary. I felt they changed their strategy a little with the transfer of Osimhen two seasons ago. I also believe Napoli will sell Osimhen soon so they will not break their salary policy for their star players. If Napoli continue their transfer and salary policies, they will maintain good finance in future but with such strategy, they will hardly win another Scudetto.
whereas for me it was actually a mistake for Napoli to sell Osimhen and this is one of their best players. but however maybe this has become Napoli's strategy to keep balancing their finances which may have a bit of a problem so they are forced to sell Osimhen to be able to get new players in the transfer market later.
losing the best players is not a good choice because Napoli without several excellent players is very difficult to win the Scudetto unless you can get other top players in the transfer market.