Turns out their formation is an "enduring puzzle in geology".

Seems the best guess is that subduction created weaknesses in the earth's crust in particular places that over time became the boundaries of the plates around 3-4 billion years ago.

This article in the Huffpost seems to suggest that where they are now is where they always were - just where the chips fell in the process of the earth settling into its skin - though it does mention: "The oldest ... evidence of subduction gathered from minerals preserved in ancient rocks... are 4-billion-year-old zircons found in the Jack Hills of Australia." Given Australia is not on a plate boundary, then maybe they may have moved - or could still form.