What if I postpone the surgery and have a car accident. Could I be found negligent for not fixing a cataract?

Yes, if you can't see well and you're driving anyway. "I have cataracts" is not going to be a viable excuse, since driving is a priviledge and not a right. A bigger question is whether your car insurance company would sue you so they don't have to pay for an accident you cause, since right now you'd probably be unable to pass the vision test of the driver's license.

And if fixing a cataract is so necessary, why isn't it properly covered by health insurance?!

Typically things that people have a high likihood of happening (such as cataracts - but also things like dental disease) but are affordable for the average person to self-pay to fix aren't covered fully by insurance - you would've been paying higher premiums all along if they were. It's similar to say eye glasses - they're important if you need them, but vision plans rarely cover them at 100% soup-to-nuts.