1-Due to automation and technology, large numbers of serfs are no longer needed, all the excess numbers are simply a drain on resources. Imagine all the trouble these billionaires go into to avoid seeing all of those plebs! I mean even mount Everest is overcrowded!

2-Things are not tidy and neat as they would like them to be. Too many people here and there and in rural areas are a lot of headaches, they will just start over with a clean plate with nicely docile types of people brainwashed correctly. Not the mishmash we have now, with muzzies, hindus, and what not, nice clean slate with an easily controllable and predictable "human" unit.

3-Religious reasons, in addition to human sacrifices, the Talmud teaches that it is a good deed to harm all non jews, there are many prerequisites to the coming of the Moshiah and the enslavement of all goys and they are just fulfilling that age old plan. (But they need the above 2 points to convince the gullible goy leaders.)