Why doesn't everyone experience a "near-death experience"?

For those that have come really close to death such as blood being lost from the body at a rapid rate, dying on an operating table, etc. how come some of those people don’t experience that peaceful and calm feeling?

Some people who have experienced a near-death claim they see heaven, see loved ones, feel calm… and then there are scientists that say it’s just the brain and chemicals.

My mom had a near death experience having my brother 22 years ago. She claims she was about to join her mom because she saw her. She said she almost died again in surgery a year ago but I didn’t hear her talk about any new experience.