There are bands with songs over an hour long on Spotify. Distrokid (the most popular service for independent artists to get on Spotify et al) has no maximum length for song uploads.

I think the real reason is Spotify pays you per "song played" not per minute played. If I listen to Dopesmoker by Sleep, which is an hour long, then I listen to a 60 second white noise clip, both artists get paid an equal amount.