Why It's Hard to Listen to Two People Talking at One Time
Developmentally, the need for bilateral brain coordination and interhemispheric pathways is plenty good reason for why some students (and nearly all young children) may have a harder time listening over background noise or focusing on a teacher's comments while a classmate is talking.
In the figure at left (see this study), see how the digit (number) recall of 6-9 year old children goes steadily down as background noise goes up.
There is an entire science of noisy classrooms - but briefly, background noise in occupied classrooms is significant (48-68 dB in one study) and it significantly affects classroom performance particularly for subjects like reading, spelling, attention, and behavior. And yes - it also affects teacher performance. Children with hearing loss or auditory processing disorders are affected more than their non-impaired peers.