Even in the storyline, they still needed to rely on their numbers to ensure the continuation of their species. Without extremely high birth rates, they would have been exterminated by other powerful races long ago.
In plain terms, they are like the rabbits and mice of the animal kingdom.
Some powerful species, especially those with long lifespans, such as various Dragon Races, Chimeras, and Night Elves, tend to have lower birth rates.
Because mathematically speaking, even if Night Elves had only a birth rate of one, reproducing once every hundred years, their population would still only increase, never decrease, due to their endless lives. Then, in ten thousand years, their population would have exploded and would have covered the entirety of Kalimdor.