The reason is simple, Osimhen's wage. His transfer fees ain't the problem, but his wage is too big. His weekly wage was 300k per-week, and adding him to the big club squad can burdening that club. It makes that club may be violating the PSR rules.
Things worsen when he demands higher wage than what he received from Napoli, but Galatasaray is the only club that can afford him 340k per week. So, that's why other big clubs moved away from him.
If the problem was only the wage than it would mean that Osimhen cares more about the money rather than glory (and there is nothing wrong with that) but it that case he could have simply accepted the offers he received from Saudi Arabia, way more money than Turkey. I don't know, maybe at the end he really likes Galatasaray, Istanbul is a pretty cool city, it makes sense but I keep wondering if there is something else. United and Arsenal absolutely need a striker, Chelsea too, and for sure they could offer a lot of money to the club and the player as well.
Bro Osimhen wanted to stay in europe, but also demands improved wage. Chelsea ever try to poached him in the past, but he rejected it as they can only offer 150k - 190k range. If he could decrease his wage, and believe me, so many big clubs would be competing
in signingto sign him. He's now the highest paid player in the turkish league.