It's called a 'post mortem' in the medical world because the patient died. It involves a high level of inspection into what went wrong and what can be learned to prevent it. I assume the term was adopted from there into project management.
It might make a little more sense in the world of shipping software in retail boxes, where products/projects had a 'done' date. The project is dead, what contributed to it's demise? Or you might generalize death into failure, and that's why we use the term instead of post-incident.
Not all post-mortems are for failures/dead products which might add even more confusion. For instance Gamasutra runs a game development post-mortem section where developers of popular (and unpopular) games can hop on and describe difficult situations they've encountered, how they did what they did, why they did that thing, etc.
It might make a little more sense in the world of shipping software in retail boxes, where products/projects had a 'done' date. The project is dead, what contributed to it's demise? Or you might generalize death into failure, and that's why we use the term instead of post-incident.