Why the result always is around 2.87

I have below code

result=0
loop_n=10000
for i in range(loop_n):
    result+=np.random.rand(3,4,10).std()
result=result/loop_n
print(result)

As I understand, if I run multi times, the result should be different because the result comes from random, but actually the result is always around 0.287

Is there some theory behind this?