Some Secret Vampire Thing

Inter-elemental pairs were hard enough to fathom but it was an established fact that all witches were female so they had to mate with other races to carry on their own. Children of witches always ended up being witches as well.

Since most races were against dating outsiders, they tended to have flings with elves until they managed to give birth and then break things off to go back to their research. If fae marriages were uncommon, witch ones were unheard of.

Elves tended to feel the same way about avoiding long-term emotional attachments, which made them the perfect way to keep the race from dying out. Fae needed witches to function in human society; they were few but important.

Witch covens were scattered all over the world and fae communities relied on them heavily. Unless other fae were content to live in the middle of nowhere and avoid humans completely, there needed to be one within driving distance to keep things running smoothly.