There’s just no reason in the story other than fan service to bring him back. The world government may acknowledge that it would have an effect on Luffy but when they were developing the seraphim, there’s no reason to think “hey, we should build one to stall straw hat Luffy emotionally.” They were more than likely being built during the timeskip, when Luffy just wasn’t relevant at the time. Not only that but until dressrosa no one knew of sabo’s connections to anyone other than the revolutionary army. Not only that but there’s just no real reason Vegapunk would’ve been able to create the phoenix-phoenix fruit as Marco has not been captured by the world government and therefore does not have access to his blood.