they must replenish their mana by releasing it from other sentient species and absorbing it. They do this through suffering

For the same reason that game meat is not the same as industrial poultry: flavor and texture.

A human captured from their free state produces an agony and a pain which are far more nutritious than those produced by a human grown in captivity and used to assist to the atrocities committed by these elves.

The effort needed to raise a captive adult is not worth the net result.

they must replenish their mana by releasing it from other sentient species and absorbing it. They do this through suffering

For the same reason that game meat is not the same as industrial poultry: flavor and texture.

A human captured from their free state produces an agony and a pain which are far more nutritious than those produced by a human grown in captivity and used to assist to the atrocities committed by these elves. How many murders does one have to see before becoming insensitive to death? Hint: not too many.

The effort needed to raise a captive adult is not worth the net result.