Why are real-time clocks always 32.768khz?

Using Arduino General Electronics
20

Let's guess how Seiko did 2 ppm Crystal Oscillator for watch at 1969, but we can not even do it for RTC at today.

my guess is;

  1. Seiko produce a lot of +/- 10 ppm Crystal Oscillators, by measure only use +/- 2 ppm one and drop the rest.
  2. temperature-compensated circuit.
  3. Wristwatch is Wrist watch. The temperature range should be ~37°C and not 0°C to +40°C.