They can afford it, but fuel and energy costs are the lion's share of their expenses, in a way they aren't in rich countries.

~1/25th of my expenses are spent on paying for energy. I wouldn't think twice about using more of it.

If 1/2th of my expenses were directed towards acquiring energy, that would be a different conversation.

And they can't just go to the forest and get some firewood. Population densities in the developing world in the 21st century are such that even if there is unowned land where you can do that close enough to where you live, it won't sustain your community for very long.

The article says "D and her kids spend another 21 hours a week collecting firewood.", effectively half of a full-time job, on top of another 14 hours per week collecting water. Where do they have the time to gather excess fuel to boil water, let alone make charcoal?