carbonic acid does form, but we show the reaction going to completion resulting in its dissociation into CO2 and H20
2 HCl + CO3(-2) --> H2CO3 <--> CO2(g) + H20(l) + 2Cl-
carbonate is a moderately strong base so it's going to abstract the protons from HCl and form H2CO3 (that's about all you need to know for the mechanism) which is in equilibrium with H20 and CO2.
we can also write it as:
2 HCl + CO3(-2) --> CO2(g) + H20(l) + 2Cl-
does that help?