Inter died when Mourinho left and they hired benitez (lol)
Juve had some troubles in recent years with being forcibly relegated and stuff. They look strong now tho and imo have recovered better than many teams would if they were forcibly relegated a division.
Milan got raped in the transfer market and simply has a weak team now
Mostly Italian teams aren't the financial powerhouses they once were and can't compete with teams like City and PSG (who can lol?). I don't feel too sorry for them though, AC Milan especially were in the past doing much what PSG is now (granted in a smaller scale) so what comes around goes around