Because he's not the player he was 2-3 years ago, let alone 5 years ago. He's not the best player in the world now, he's nowhere close to it. He could probably be the best player in the MLS, or the best player at Sporting. But he's not even in the top 10 players in the Premier League, hell - I'm not even sure he's one of the best 5 forwards at Utd.
Obviously he was a great player, one of the all time greats, and he's still a good goal scorer. But he offers literally nothing else on the pitch. His work rate is appalling, he doesn't link up play well, he's not creative, he won't close down or harry defenders. He'll try and score at every opportunity - and will want and expect our attacking play to be set up in order to provide him with goalscoring opportunities. Because he hasn't got the ability or the legs to make goals out of nothing