Imagine looking at a fire, let’s say a candle flame. There is a region with partially ionised gas at the edge of the visible flame in the lower part of the flame. Think about the gas in that spot. It is a mixture of air from the surroundings and evaporated wax that came from the wick, undergoing chemical reactions that heat it up. The crucial issue is that the gas is not at rest, but moving upwards through the flame at significant speed.

Where were the gas molecules that are reacting with each other in the flame right this instant a fraction of a second before? Some of them were in the surrounding air, being drawn toward the flame, and some were inside the wick in the form of liquid wax which then evaporated. And a fraction of a second later, they will be rising up above the flame and no longer emitting noticable amounts of light.

The atoms and molecules which are ionised at any given time are so only for a short interval of time. The gas inside the part of the flame where there is significant ionisation is constantly moving and gas which moves out of this zone is being replaced by other gas moving inside it. This means that the duration of the ionisation is also quite short in the case of the flame – when you look at it from the point of view of the gas molecules – similar to the lightning case.

The difference is that in the case of lightning, the whole gas volume is ionised almost at once, then it recombines and the process is over, whereas in the case of a flame it’s a steady-state process with fresh reactants moving into the flame and reaction products moving out continuously, but the duration of ionisation of _individual molecules_ is also short, as explained in the [answer](https://physics.stackexchange.com/a/694490/151244) by [honeste_vivere](https://physics.stackexchange.com/users/59023/honeste-vivere).