Because the accuracy of an estimated quantity mostly depends on the size of the sample, not on the size of the population [1]. This does require assumptions lik

Because the accuracy of an estimated quantity mostly depends on the size of the sample, not on the size of the population [1]. This does require assumptions like somewhat homogenous population and normal distributions etc. However, these assumptions often hold. [1] https://stats.stackexchange.com/questions/166/how-do-you-decide-the-sample-size-when-polling-a-large-population https://stats.stackexchange.com/questions/166/how-do-you-dec...