Actually, it does change the pH; it's the matter of
how it changes the pH.
This is a quote taken from the
Skeptical Aquarist (love these names):
Dissolved carbon dioxide is correlated with the pH scale. The carbon dioxide dissolved in water has even more effect than the oxygen. Oxygen remains as an O2 molecule, whether it's in its gas phase or in solution, but when CO2 is dissolved in water, a small proportion of it reacts chemically with H2O to form carbonic acid, H2CO3. (There's no mystery about that: just add up the six atoms.) In water carbonic acid dissociates rapidly to form a H+ ion and HCO2 (bicarbonate), so it affects the carbonate equilibrium, and pH values change as a result.
I won't argue that it's quite possible for roots to take in CO2. Anything's possible. I'm just saying that, for instance, if I wanted vitamins from a tomato, I could take that tomato and rub it all over my near Brad Pitt-like chiseled naked body and hope my body absorbs those vitamins through my skin; or I could just eat the tomato and be done with it.