Alleria was able to harness and use the void before the events in Argus. Her consuming the void revenant and then L’ura made her powers much greater, but that does not mean every other void elf should be on the same level. Maybe hunting down and consuming void creatures could be the path of the void elves, with prestige ranging on each one’s success in getting more powerful.
Besides, it’s not like the canon void elves have any known way of creating more of themselves anyway. Or, at least, there’s none shown in the actual story so far.