Why is the population's standard deviation the same as the sample's standard deviation?
What you mean is why is the formula for calculating the population SD the same as that for calculating the sample SD.
That is because that is how the SD of a data set is defined (the population is just a set of numbers as is the sample).
You have a different formula to calculate an unbiased estimate of the population SD from a sample.
(note the difference between continuous and discrete distributions is being glossed over in the above)
CB