some of them do look fancy, and a bit complicated? not a good effort/cost ratio, where the hassle of making something outweighs the cost of buying one. but when you put in the fun factor of making your own stuff then it all makes sense.
i tried a WWII afrika corps style stove once where you mix gasoline and sand or soil into a thick sludge in the bottom of a tin and it burns long enough to boil your coffee water. it does work quite well but i lost my eyebrows lighting it, and it made everything stink. i think modern petrol has a lot more additives than 60 years ago!!
i may have a go at the coke can one, not from any practical need but it look interesting to do, like wittling a bit of wood