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".

Apart from that the population may be too big or part of it somewhere hidden, part of objects just may not exist at the time of the measurement.
If you craft bolts and measure them, statistical estimates like average length plus minus remain valid for the bolts you will make in the future, if you do not change the technology or intended length.
It is also possible to imagine situations when some objects are already lost/destroyed at the time of the measurements, and you need to make conclusions about them from the objects still available.
