I just tried Fizzies for the first time about a month ago; my mom told me about them when I was a kid, describing them as some kind of prehistoric relic like the straws that flavored milk or Koogle, so I was rather surprised to see a point-of-purchase display of them at a restaurant gift shop. The root beer tablets taste vaguely like flat root beer, but visually it’s offputting, like a cloudy cream soda. I haven’t taken the next step, though, and tried popping a tablet in my mouth, which was apparently the “cool kid” way of consuming Fizzies when they were new.
I forgot about Airborne and Emergen-C being similar. I had some Emergen-C a few times (I was at a hotel that put packets of them on my pillow like a mint, oddly) and was underwhelmed by the fizz and bicarbonate taste there, too. I think your observation is right, that the method simply can’t provide enough carbonation. A soda has carbon dioxide dissolved in the water, so there’s quite a bit left to replace that which bubbles out, while the mixes at best will resemble a soda where all of the gas has come out of solution and bubbled out.
Going back to the longer discussion of colas and root beer needing carbonation; while I agree, I do think it’s interesting that both drinks started life as non-carbonated drinks (root tea in the case of root beer), and the general cola and root beer flavors aren’t disagreeable in hard candies, gums, lip balms, medicines, or other products… but when it’s in a drink, we suddenly need bubbles. Fruit drinks are great carbonated or not, so why do these (and other fantasias) need carbonation?