I know what you mean. Prolly coz they were made as films first. I love all those Gene Kelly MGM musicals (particularly 'It's Always Fair Weather') but only ever think of them as movies. (I did have the misfortune to see Tommy Steele in 'Singing In The Rain' once. Never again.) My favourite musical is 'Fiddler On The Roof'.
*slinks off before someone picks up on a tendancy towards show tunes*