“But I would think loss of vision has to be due to something.”
Of course it is something and I am surprised your vet didn’t mention this.
Just like humans.
https://vcahospitals.com/know-your-pet/cataracts-in-cats#:~:text=When%20a%20cat%20develops%20a,age%2C%20sex%2C%20or%20breed.
She didn’t have cataracts. She didn’t have cataracts or glaucoma and the blood vessels behind her eyes were fine. All the typical things that would cause a loss of vision.