HI there,
effervescent tablets are made in a specially controlled low humidity environment and they start out as a dry mixture of powders. This mix contains the active ingredient and sweeteners etc.
The powder is then compressed in a heavy tablet punch to form a tablet shape. So no fluid is actually used in the process, and that's why they don't fizz during manfacture.
Oh, also, the ingredients that cause the fizz are usually called carbonates - i..e sodium carbonate.
Hope that answers your question!