Why don’t we calculate the average of an entire given population instead of computing confidence interval to estimate the population mean? We compute confidence intervals to estimate the true population mean of either a sample (when population standard deviation is unknown) or an entire population (when population standard deviation is known). But I wonder why, if we are given an entire population, why don't we calculate the average of that population instead of computing confidence interval to estimate the population mean since we already have the entire population? I'm not a Math or Stat major so I apologize if my question does not sound "smart".

The other answers are good, but to give a concrete example: suppose you want to know the average height of mayors of London. So far there have only been 3 mayors of London, so you take the mean of their heights and find it is 173cm. This is exactly the mean height of all the mayors of London so far. But there will be more mayors of London in future, and they will shift the mean to different values, so it isn't really useful to claim that the population mean is certainly exactly 173cm.
