It makes a lot of sense for an audio file format to support multiple tracks when each track is intended to target a different output device (left and right speakers, etc.)
It makes less sense when the intent is to mix those tracks together and send them to a single output device. Producers and audio engineers want full control over that mixing process because it's almost never just a simple sum. They are doing audio compression (sometimes multi-band), dynamic EQ, saturation, limiting, etc. They wouldn't want to give that up, because it's an essential part of making a good sounding recording.
It makes less sense when the intent is to mix those tracks together and send them to a single output device. Producers and audio engineers want full control over that mixing process because it's almost never just a simple sum. They are doing audio compression (sometimes multi-band), dynamic EQ, saturation, limiting, etc. They wouldn't want to give that up, because it's an essential part of making a good sounding recording.