Dark elves, a vampire-like species, exist on a plane known as the fae-realm and must feed on other creatures to survive. Their souls are constantly being drained of mana by their deity, who ironically despises them. This makes them old and decrepit, and slowly drives them mad with starvation. To avoid this, they must replenish their mana by releasing it from other sentient species and absorbing it. They do this through suffering. Other races constantly replenish their own mana naturally, and the dark elves have developed techniques and technologies to keep their prey alive for centuries or even millennia to get the maximum amount out of them.
These elves occasionally raid the mortal realm in an event known as "The Wild Hunt", in which they capture alive as many different sentient species as possible and drag them back to the fae world to become slaves or to be tortured. These raids are often successful, as they are unpredictable and far more advanced as a civilization than others. Elves do not reproduce naturally, and must use artificial wombs to grow new children. They do this to replenish their own numbers and their slave stock. It is a cheap and inexpensive method, and people are grown in batches to save time.
Since they have the technology to literally grow new slaves, it stands to reason that they would be able to use this method to avoid having to risk their lives in raids to bring back people to torture. Why would this not be the case?