Terminology is important here. Fire brigades log lots of "unexplained fire deaths", many of which are people who have turned into piles of ash for no apparent reason, often without nearby flammable materials catching fire.
The idea of "spontaneous human combustion" is not really an explanation, because it is not a theory of HOW people burn, it just vaguely implies that people burn without an external source of ignition. In much the same way "UFO" simply implies that a strange unidentified thing in the sky was a flying object (not a reflection, Venus, hallucination etc.)
Charles Dickens and others suggested long term alcoholism as an explanation, but in reality people would be dead from alcohol poisoning long before their body alcohol levels made them flammable. There has also been speculation about the liver generating enough heat to ignite a body.
Whatever we call it, people sometimes burn for no obvious reason in very localised fires, and saying "it's impossible, so it doesn't happen" is being willfully blind. In only one or two cases have the people actually been seen burning. In many cases they seem to be elderly people living alone, receiving few visitors, so the process of burning could have taken many hours.
The wick explanation is the best, but it still leaves something to be desired. This theory says that clothing smouldering from a dropped cigarette etc. can draw out and evaporate water from human tissue, causing it to slowly burn. So, for instance, an elderly person has a heart attack or stroke while smoking, dies, and their clothes and body slowly burn.
However, not all unexplained fire deaths fit this pattern
In one case a fireman reported training his hose into the stomach cavity of a human body, from which flames were blazing.
There may not be a single explanation for all these fire deaths. "I don't know" is a perfectly logical response, better than jumping on the first theory which sounds vaguely rational if you ignore a few details.