I think I know the real reason.
Which is?
Alcohol in the dust clouds. Goddamn stuff is everywhere. Any lousy species ever invents the telescope and the spectroscope and starts looking in between the stars, what do they find? He knocked the glass on the table. Loads of stuff, but much of it alcohol. He drank from the glass. Humanoids are the galaxy's way of trying to get rid of all that alcohol.

Chapter IX (p. 148). - Culture series - Use of Weapons (1990)