As I recall, people were willingly amputating limbs and replacing them with mechanical parts. This likely led to the Wardens' gradual accumulation of cybernetics, until very little humanity remained.

[Akechi pauses, frown deepening.]

Not that there's anything wrong with being mechanical, of course, but their programming seems to override all reason and empathy. Donatello-san already tried analyzing their CPUs and found little beyond their primary objective to eliminate us.