Something in their nature probably makes them reproduce less. No one is an elf in real life, so we will never know. Lol. But humans are the only species on earth that enjoy six (yeah i said six). For animals its just insinct and they get no joy out of it. Maybe that's how it is with elves. Maybe they only have six for reproduction. This means that since they don't care much for it, then they don't do it that much (less babies). But I do agree with you that it's kind of weird. Shouldn't there be elves from the beginning of time around? How come we don't see any? If there are, they can tell us about major parts in the past that we know little about (how elves got here in the first place). It's a rather big inconsistency.