The reason is that these clubs are still hesitant to bring in Oshimen because his price is very expensive and his salary is very large. If Napoli set Oshimen's price at no more than 80 million euros, clubs like Arsenal, PSG or Chelsea will definitely scramble to bring in this player.
Arsenal will not pay that expensive cost, PSG might not be ready to spend it with their strategic transfer changes last year. Chelsea can be a possible team to make that offer and buy Osimhen from Napoli. Another candidate club can be Manchester United if they think they need a better striker than Højlund. Liverpool don't have abundant transfer budget to join this race. Choices for Napoli and Osimhen are actually limited and I believe Napoli know that a longer time they keep Osimhen in Naples, a more disadvantage they will have in future when selling the striker.
I know Oshimen is a good striker. He has contributed a lot to Napoli but there is no guarantee that his performance will remain good in the next season because his quality has decreased this season. If Oshimen's performance continues to decline, the clubs that have bought Oshimen very expensively will experience losses because their selling price will also decrease.
At the 25-year old, Osimhen will have like next 3 to 4 years to improve and reach to his best capability of performance. Besides the current contract expiration date, the potential of improvement and ageing effects on the player, will be two biggest factors for any big club that are considering to spend big money for Napoli and Osimhen.
The contract will be expired in 2026, next two years and it means we are counting down till a departure day of Osimhen with Napoli.