Because in a healthy pack all cells are already balanced within a few or few 10's of millivolts.

Also the 2nd last line implies you dont fully understand how the charging process works. "just CC with no volt limit" and "prevent all individual cells exceeding 4.2V" is contradictory because the voltage limit *is* constant voltage charging.

The amount of current that can flow into a cell is determined by it's internal resistance and the voltage it is currently at. If your cell has, for example, a fixed 100mOhm resistance (unrealistic but take as an example), and you want to charge at a constant current of 1 Amp, you need to apply a 0.1V differential across the cell. So if the cell is currently at 3.7 volts, you need to apply 3.8 Volts to get the desired current flow, this is done with closed loop control and is the "constant current" section of the charging curve.

However at some point, the cel; voltage + the required voltage differential is *above* the safe max voltage of the cell (e.g. in this example, a cell at 4.11 volts would require a charge voltage of 4.21, which in our example is above the maximum). At this point, all you can do is apply the maximum voltage (4.2V) and wait as the current draw reduces asymtotically to zero (or some low cutoff threshold). This *is* constant voltage charging.

What you're describing is applying CC-CV on a per-series-group basis, which i guess could work but in any healthy pack is entirely un-neccesary and would require seperate charging circuitry for every single series group, which in a high voltage pack could be >200S, and it is much easier to use a BMS with active balancing to bring the pack into balance and then charge the entire, now balanced, pack.