What I've always found weird in gamedev is that everyone under the sun builds their own graphics engine - I don't think there's even popular middleware for that

What I've always found weird in gamedev is that everyone under the sun builds their own graphics engine - I don't think there's even popular middleware for that. But almost nobody builds their audio engine - to the point that basically there's no good open source audio engine in existence. I wonder why is that - while the requisite knowledge is not as readily available as it is for graphics, and this might be hubris speaking, but it can't be that much more difficult.