why ethanoic acid when reacts with sodiumbicarbonate forms CO2 and water rather than carbonic acid ?

See carbonic acid is the reaction product of water and carbon dioxide and exists in an equilibrium with water and carbon dioxide whenever the latter is dissolved in the former.

H₂O (l) + CO₂ (g) H₂CO₃ (aq)

In this reaction a majority of the carbon dioxide is not converted into carbonic acid and stays as CO₂ molecules. Also, to increase the concentration of carbonic acid, the pressure has to be high.

Thus, in the reaction mentioned in the question, we get CO₂ and water.