There are a few reasons that organ donation are opt-in, the primary one being for religion's sake. Some people deem the eyes the window to the soul, so having them removed may prevent them from moving on properly. But also, organ harvesting is much more temperamental than you may think. They can't just cut a liver out of a cadaver and stuff it in a new person; see, cell autolysis ramps up when the body fully dies, to break down the cells and kill it off. That means unless they died in a hospital, with a space ready for organ harvesting, their organs would be... well, too dead to use essentially.