That's because you didn't call randomize() :)
If you check the first example in https://nim-lang.org/docs/random.html , it says:
# Call randomize() once to initialize the default random number generator
# If this is not called, the same results will occur every time these
# examples are run
randomize()
From the random module docs:
import random
# Call randomize() once to initialize the default random number generator
# If this is not called, the same results will occur every time these
# examples are run
randomize()