Well, anonymous, in most cases, it’s simply because fire-types don’t live in forests. Most of them are actually native to places that are better suited to withstanding fire (and lava and other heat-related problems) such as caves or deserts. Charmander in particular are only found in tropical volcanoes, if they aren’t simply bred in captivity. Likewise, a few of the other starters, namely tepig and torchic, only exist as domesticated, bred species, so their habitats are usually well-maintained, well-irrigated ranges.
Ponyta, meanwhile, are capable of controlling the temperature and height of their flames shortly after birth, which means that while brushfires are still a risk in their native grasslands, they tend to be less common than one would think, thanks to the superior control the species has over their abilities. It also helps that they’re well aware that burning their only source of food would be a bad idea, so it’s particularly imperative to them that they exercise such control.
As for the pokémon that do live in forests, most of them also exercise extreme control over their abilities. This goes especially for predatory pokémon, such as vulpix, growlithe, houndour, litleo, and fennekin. All of these species rely on biting and clawing tactics, rather than their fire abilities, to hunt, and as such, ranged fire attacks are only used in extreme defensive measures, such as to repel invading humans.
However, concerning the remaining—fletchinder, chimchar, pansear (despite what the Unovan pokédex states), and cyndaquil—those do start about as many forest fires as one would think. Luckily, almost all of these are rare, except for fletchinder and its normal-type pre-evolution, which is why this author for one is baffled that Kalosean forests and grasslands still exist.
The Pokédex According to Bill 