Here is why.
Your ears respond logarithmically to sound intensity, as measured in decibels. A 3 decibel increase in sound strength requires a doubling of the power of the sound source, and that +3dB increase happens to be just barely perceptible to your ear. Furthermore, if you double the distance between a sound source and your ear, the intensity of the sound is decreased by 6dB.
These two facts mean that in practice, the closer of two equal-strength sound sources will be strongly favored by your ear: we say that closer sound source masks the distant one and if the difference in sound pressure levels between them is 6dB or greater, the weaker of the sources becomes impossible to hear.