His statistics with Napoli have tended to increase since he joined, peaking in the 2022/2023 season when he could become a goal scoring machine for Napoli, last season he declined again because there were several problems he faced, one of which was injury and also the overall team play Napoli also declined slightly, so that affected Osimhen's performance too. I understand why Conte didn't include his name in the list of players, because every coach must have their own criteria, and Conte prefers Lukaku.
You are right and if you look at it overall Osimhen's performance has indeed improved, but last season he had several problems that resulted in a decline in performance. A coach has a view of the players he likes and maybe Lukaku is a player he has been with so he understands his form of play better and chooses Lukaku over Osimhen. Moreover, the drama that happened to Osimhen which never ends so that when Conte uses his services it will slightly affect the consistency in the form of the game because he is no longer wholeheartedly playing for Napoli.