Why couldn't Beethoven or Mozart write a symphony or concerto that's 'gloomy'/minor key/dark from start to finish, like 2nd mov of 7th symphony or 1st and 2nd of 3rd symphony, or 1st of 20th concerto etc.?

I'm here listening to Beethoven's 7th and being disappointed by the 3rd movement because it absolutely does not follow the great 2nd mov emotionally - which is one of the greatest thing ever written. Why the constant mood swings? Ugh.