The citizens of Norrath have also developed a number of trade skills over the course of their history, as would any civilized world. Merchants on all continents buy and sell components to supply those who opt to learn these skills, be they apprentice or master. Some learn the skills to feed themselves, to earn money, or simply to provide themselves with items they would otherwise lack, due to their cost.
Note
You can always raise a class-specific or race-specific skill (Poison Making for rogues, Alchemy for shaman, Tinkering for gnomes) to its maximum if you are the right class or race. Also, taking one of these skills over 200 before a general tradeskill will not prevent you from later taking another skill over 200.