I personally believe, this problem is because Conte does not want Osimhen and then, Osimhen is available for sale but at the same time Napoli still want Osimhen to stay. Because anyway, with the Napoli factor extending Osimhen's contract last season is also at least proof that Napoli want Osimhen to stay at Napoli longer. And also, last season also at the time of the contract extension, that Osimhen did prefer to stay at Napoli rather than leave.
Osimhen is important with Napoli and Conte actually likes traditional strikers in his previous clubs. He probably likes Osimhen too and the problem is that Osimhen no longer wants to play for Napoli and waste his time here. Perhaps Conte understands it and knows that he won't be able to convince Osimhen to stay. Perhaps he already tried to convince the player but fail so he and the club agreed to put Osimhen for sales on player transfer market.
Because anyway, last season also the fact that several elite teams have been interested in buying Osimhen but anyway, because Osimhen and Napoli have agreed to continue to cooperate, and then the contract extension also occurred. But unfortunately, Napoli's performance also declined and Napoli appointed Conte as Napoli's new manager as well as the fact that Conte preferred to buy Lukaku rather than keep Osimhen. Thus, I believe that if Conte had not made such a statement, then maybe Osimhen would still be fine to be at Napoli.
Lukaku and Osimhen are very similar strikers so if Conte like Lukaku, he does like Osimhen. Selling Osimhen and buying Lukaku will help Napoli and Conte have a good extra money to buy one or two more players. It is helpful budget for them to improve their squad before a new season.