The point is that if you keep your water levels up, you don’t lose salts, ie you only need plain water.
This isn’t exactly accurate, although there is some truth to it.
Unless you have certain medical conditions, sweat will always contain some amount of sodium and other salts, along with a bunch of other chemicals. Studies have shown that intensity of exercise – independent of sweat volume – also has an impact on the amount of salts in your sweat.
To OP’s question – I’ve often wondered same, but find that Nuun tablets aren’t too expensive, and the slight expense overcomes my laziness at trying a DIY solution.