Tepig can not truly be said to be extinct in the wild, for the term "extinct" implies that they were ever extant there in the first place. These pokemon descend not from some fiercer, wild equivalent species, but are the result of Unovan farmers crossbreeding Grumpig with a variety of fire pokemon over a period of many generations, until they succeeded at creating a fire type pokemon which produced a Grumpig-like meat.
Grumpig meat was eaten in much of the world before the advent of the Tepig, and is still treasured in some places today, but cooking it is a long process which required a great deal of tending to fires, and could be dangerous in the era of wooden housing. Tepig meat, however, is cooked throughout the life of the pokemon (and for some time after death) by its own internal flames, and has long been popular as a meal because it has a pleasant taste and takes virtually no time to prepare; even in this era, when foods which most be cooked are often abandoned in favor of packaged foods, Tepig meat remains popular among many individuals.
Many who have prolonged experience with live Tepig, however, refuse to eat these pokemon. This is not because of their cute appearance, but their mannerisms; they are as loyal as a Snubbull, as friendly and trusting as a Growlithe. Some farmers, who had slaughtered many Tauros or Grumpig without flinching, quit raising livestock altogether after doing likewise to their Tepig awakened in them countless moral qualms. Once they quit, said farmers typically donate their remaining Tepig stock to beginner pokemon trainers, both to protect the lives of their remaining stock and because they are convinced that no one who raised a Tepig for battle would ever eat that pokemon's meat again.