The jungles in Vietnam and Korea back during the wars posed a problem to U.S soldiers because of Guerilla Attacks. Yet during both wars the use of napalm was Common. So why didn't they use the napalm to burn down the Jungles? They used napalm on Enemy Sites and Camps to clear them out so It can't be the "Oh but they will be burned alive" excuse. Any reasons as to why this was not used?
Well lets turn this around, why do you think that napalm was not part of deforestation efforts?
Especially in Vietnam there were vast and prolonged efforts at reducing forest and crop cover that aided NVA and Vietcong movements. However napalm was not the primary tool in this effort, but instead the Chemical Corps developed a series of color coded chemical defoliants that could be sprayed from aircraft or vehicles over a selected area to kill the plants. This offered an ability to target wide swaths of terrain, but also not to risk the dangers from fires that could get out of control. In particular the infamous Agent ORANGE was used in massive quantities despite its carcinogenic components to do everything from kill food crops to cut back plant growth around US bases.
It boils down to basically that they had specifically crafted tools for deforestation efforts and that were not just "drop enough incendiaries to burn it all down" though that was part of the strategy at times.
[This piece gives a rather good overview of the deforestation efforts in Vietnam, named Operation RANCH HAND, including maps of where defoliants were used]http://www.agentorangerecord.com/images/uploads/Stellman1537(2)(1).pdf)