Mythical lineage— not to be confused with genetic lineage— is a process of magical transformation where ordinary living things are given access to mythical abilities. This only occurs with the patronage of a mythical creature, one who has decided to pass on the legacy of their magic to a new generation. Most mythics cannot give birth, so lineage is a chance for them to adopt one. Creatures who receive a mythical lineage are known as heirs.
Since the goal of lineage is essentially to have a child, the role of heir is not always beneficial— at least, not at first. The changes are often disorienting and severe. Humans who get caught up in the process might not even know what's happening to them, as they awaken to supernatural powers and eventually shed their human form completely. Most mythic patrons will help their heirs out in some way, teaching them how to use their new abilities and guiding them through the transformation process. Sometimes, though, things go wrong. And to a human who has been left to fend for themselves, lineage might seem more like a curse than anything else.
Lineage can only be passed onto something that does not have magical powers of its own. The conflicting sources of magic would tear the would-be heir to pieces. In theory, heirwitches could exist— and they would be extremely powerful— but the odds are simply too low. Instead, mythics rely on the tried-and-true method of selecting heirs with little to no potential. It would be even better to find one that knows nothing about magic at all, someone who idealizes it as fiction or fantasy, so that they can find excitement in being a part of something like this.
Therefore, humans from our world are very desirable heirs.