The difference is that each ion could also occur from a different source salt besides sodium chloride. In a system with only pure water and pure sodium chloride, your proposal would work.

You could have e.g. sodium chloride and sodium carbonate in there or more likely sodium citrate, since you said sports drink. Those sodium ions are the same and indistinguishable, regardless of source. People who have to limit their sodium intake should not need to calculate the sodium content of each salt and then sum that up. Instead you get one sum value directly, which people can interpret much better.

In addition I think the reason is that they simply make a ICP-MS measurement and get one sodium cation value and the same for the other ions. Therefore they cannot combine the values,to a salt because they didn’t determine the sodium chloride salt content, only sodium cation and chloride anion content separately.