That's actually a pretty answerable question...they move in miniscule, calculated, increments (degrees) and have created and maintained the conditions within humanity (fear, doubt, confusion/distraction) which have allowed, and continue to allow, their progression...is the answer to that

I feel the "why" is sooo much more important/relevant though

Aand I've found that answer as well...their own enslavement, is the only reasonable explanation

Sadly...they were victims 1st/too