That's why you should read about CFR, the calculation method used by most organizations to reach a mortality rate of a virus (flu or COVID-19). It is based on extrapolating known data to reach estimates on total number of infected. It is a used method and reaches some conclusions. However, it also results in a wide data spread (hence why flu estimates range from by tens of millions for infected each year). Furthermore, CFR almost always overestimates the mortality rate of a virus when it is first spreading.Consider also that Italy is unique in that many working adults live with older parents. Also we don't know how many people had the virus but didn't know it and/or didn't report it so their survival is not part of the death rate calculation.
You should check out some reading on this, esepcially scientific papers on the CFR. Some good places to start:
Since December 2019, when the first case of 2019 novel coronavirus disease (COVID-19) was identified in the city of Wuhan in the Hubei Province of Chi...
I want to be clear and say CFR is not a fool's errand. It is a methematical principle and there's a reason why it is used globally. Still, even those orgainzations that use it know it has a wide scope for bias.