Adding to your question, why did the liver evolve to regenerate while other organs evolved to have spares? What makes it special?

The liver is responsible for clearing out the majority of toxins and since many of them can cause cell death, the organ needs to be able to regenerate itself.

I’m guessing it has something to do with the size of a liver. Organs that have spares are relatively small.

Lungs? They’re pretty big. My grandfather lived 60 years with only 1, and it stretched to fill most of his chest cavity.

a plastic surgery to build in Klingon like capabilities - take the piece of the liver like for transplant and transplant it into the other side of the body, and after some time for regrow the person would just have 2 full livers.