The wines are bottled as early and as quickly as possible to preserve as much CO2 as possible. This isn't fizzy CO2 bubbles though, this is CO2 that has dissolved into the wine. In winemaking, as yeast eats up the sugars, it naturally creates alcohol and CO2. If a wine has too little CO2 dissolved inside it, it can feel flat. The CO2 increases the perception of freshness and acidity, creating a more lively wine.