The brain adjusts to help us when we are looking at something intently.

In Brain Science, Senses and Perception by Brainy Days Ahead

Researchers from Linköping University determine that when we are focusing in on something that requires the use of sight, the brain reduces hearing to make it easy for us. In fact, at any point in time, only one of our senses can perform at peak strength as the brain adjusts to allow us to concentrate.