The reason is that TFR is not the average number of children per woman over her lifetime. Instead, it is calculated by measuring the average number of children

The reason is that TFR is not the average number of children per woman over her lifetime. Instead, it is calculated by measuring the average number of children born to women at a specific age (e.g. 15, 16..49). Adding all of these up results in the TFR. Thus mortality rates are not included in the TFR.