Half of these people are entitled or forgetful or lazy. The other half are functionally illiterate, and they have been muscling their way through life using whatever means possible without the ability to rely on their reading skills.
They probably find it less embarrassing to blame their eyesight than admitting to not being able to read to a sufficient level.

(I quickly googled the numbers, and the amount of people who can’t read or can’t understand what they read is astonishing).