The same happens when we follow the directions of a passenger navigating for us. My bet: part of us is concentrating on hearing instead of seeing. We've also delegated navigation to somebody else and that impairs our navigation abilities until we're back doing it ourselves.

"Regulatory and warning signs should be used conservatively because these signs, if used to excess, tend to lose their effectiveness."

-- Manual on Uniform Traffic Control Devices, 2009 edition, section 2A.04, paragraph 01

Replace 'signs' with anything--no one's patience/attention will hold up in the face of a constant barrage of information.