the simple answer is because it was functional at the beginning. and become pseudogene after a while. thhere is no bad design here at all, just a natural degeneration. if we will find a car with a broken mirror we will conclude that the car had a full mirror at its begining. the same is here.
by the way: no one can prove a bad design. for instance: at the past evolutionists always claimed that the retina is an example of a bad design since its in the “wrong direction”. now we know that its actually a great design:
“Having the photoreceptors at the back of the retina is not a design constraint, it is a design feature. The idea that the vertebrate eye, like a traditional front-illuminated camera, might have been improved somehow if it had only been able to orient its wiring behind the photoreceptor layer, like a cephalopod, is folly”
or:
Phys. Rev. Lett. 104, 158102 (2010) - Retinal Glial Cells Enhance Human Vision Acuity
“The retina is revealed as an optimal structure designed for improving the sharpness of images”
or:
“IT LOOKS wrong, but the strange, “backwards” structure of the vertebrate retina actually improves vision”