I think (I'm no expert) 1000 made sense at the time. All of measurement seems to be more or less made up on the spot, and everybody agrees to stick with those units. Nice short writeup of the history of length here[1]. I'm not knocking metric, and I agree 1000 is nice. From our current understanding of everything, it all holds together really well. But I think that was pretty true of all units of measure that lasted any length of time.

1 https://math.ucr.edu/home/baez/inches.html