Several teams did anyway. @devilish makes it sound like they dominated the football world with ease, but they needed their fair share of luck to win the two European Cups. They were pretty much out of the European Cup in 1988 against Belgrad, had a player sent off and were 1-0 down in the return leg (the first leg ended 1-1). Then fog came up, so thick, that the game was called off and replayed the next day. They got a new chance with 11 men, it ended in a draw and Milan went through on penalties. They could have easily gone out against Heynckes' first Bayern side in the semifinals in 1990 as well, both teams created enough chances to win the tie and in the end Milan went through on away goals in extra time.Sorry to say this but I think the Liverpool side of the late 80s would have given that Milan side a hell of a game.
They never won the league and the European Cup in the same season either and while Serie A was clearly the strongest league in the world back then, there were several other fantastic European teams, that might be signficiantly underrated today. I'd argue there were as many, if not more, real top teams than we have today, because the foreigner rule prevented that smaller leagues lost all their top players early. That Belgrad side, that should have kicked Milan out in 1989, kept their team together and went on to win the European Cup in 1991. A truely great side, fantastic to watch.
That being sad, it's hard to argue against Sacchi's Milan team being one of the greatest, if not the greatest, team of all time. In full flow, they were the best I've ever seen. They could starve you off possession, they could sit deep and counterattack and they were strong and physical and could bully the opponent if they had to. In a way they were just flawless, something that you probably can't say about Pep's Barca, even though Milan was less dominant, less consistent.