It's not like they really even reflect what are later considered the greatest movies to come out that year either, though.
This is true. Don't think any awards have been consistent with this though. Looking at each decades this is how it looks in my opinion, not my opinion of the movies but how they are generally regarded critically as i understand it. Anyone correct me if i'm wrong about something.:

1920s/30s (there's only 2 winners in the 20s so i grouped them in here): 4/12 seem to be generally regarded as among the best of their time, with some others on the outskirts. Wings, All Quiet On The Western Front, It Happened One Night and Gone With The Wind. Mutiny On The Bounty and You Can't Take It With You seem to have a good reputation too although the latter may be largely because it is a Capra film that won best picture. The rest are forgotten.

1940s: 4/10 with a few others having a pretty good case. Rebecca (starting to get a better reputation in recent years as one of Hitch's best), How Green Was My Valley (obviously it beat Citizen Kane but regardless it is still regarded as one of the best films of it's time/all time by many), Casablanca and The Best Years of Our Lives. The Lost Weekend, Hamlet and Mrs Miniver are also well remembered. The rest are forgotten but i'd say that's a pretty good decade.

1950s: 5/10 All About Eve, From Here To Eternity, On The Waterfront, Bridge On The River Kwai and Ben-Hur. Marty is also fondly remembered.

1960s; 5/10 The Apartment, Lawrence of Arabia, West Side Story, My Fair Lady (not sure about this but it does usually seem to turn up on best american musical lists) and Midnight Cowboy. In The Heat of the Night, The Sound of Music and Oliver are also remembered.

1970s: 8/10. I'd say to some degree all of them except The Sting and Patton (although they are still well remembered) are seen as among the best american films of the 70s.
Are you kidding me? The Sting is definitely one of the best movies of the 70s. It's an awesome film.