It's not a deep hole. It's a misunderstanding on your part. For more than 30 years there have been laws allowing those with significant vision impairment to use assistive devices to drive. Many states require specific certification and training to use these devices. These assistive devices aren't as practical to use all the time, so they aren't worn when they aren't driving. I don't think this is common so I doubt it's clogging handicap spaces.
I am sure that these people would love to either have someone to drive them around all the time or be able to have all the time correction of their vision, but neither of those is possible for everyone. Hopefully, permanent correction will be available for more people eventually.
What are you proposing? That these people shouldn't be able to get a driver's license or that they shouldn't be able to get a handicap placard? Again, if you have a problem with the laws, you can work to change them. Otherwise, I am just explaining the way it works.