Backing up for a second: One of the functions of the setting is that wild magic naturally ebbs and flows through the world. Where it gathers and wells up, this is called a Nexus. A Nexus can produce spontaneous magical affects, including the creation of living beings.
Goblins are one such example. Really it's a catch-all term for a number of creatures that are "distorted echos" of humanity, miraekind, or other races made animate by magical surges. Unlike other periodic surges which have warped living populations, goblins seem to emerge directly from the land itself. They carry twisted elements of others' societies, seemingly rearranged at random by the magic which conceived them.
Unfortunately, once they emerge into existence, crawling from trees, fens, and the earth, they're also cunning enough to hang on in the long term. While Goblins can reproduce the good ol' fashioned way (and do so, frequently), without more emerging from nexuses they are not likely intelligent enough to hang on in the long term.