Why don't car batteries have a better state of charge indicator?

Did you ever try? How long does your coverall stay white when servicing cars?

Actually, no.

You can not read how high a ball floats. But it is very easy to read the scale in a graduated hydrometer, which is designed for that.

For which you need to read the density in all cells and do calculations.

You need to read the charge in every cell. If the cell that happens to have the balls say "green", 100 charged, but there is another cell with no balls that says "red", 10% charge, and the other 4 cells you do not know, but happen to be 80% charged, then the actual charge of the battery is equal to the charge of the worst cell, ie, 10% (depending on the intended usage).

However since a battery is an integrated single unit these days, who cares what the individual cells are doing? One tests the whole battery and replaces it as necessary.

The load testers test the battery by applying a small load to it , measuring the voltage drop, and inferring a voltage drop at the rated Cold Cranking Amps. That's all you need to do.