It is also the other side of the coin: Why are non-arid soils so acid? Because the constant rain (water) acidifies with the uptake of carbon dioxide from the atmosphere forming a weak carbonic acid that drops the pH to around 5.5.
I remember a different explanation I received form school. Something like this. I think this is something else and not just a different way of saying the same thing.
The majority of acid soils, however, are the
result of a combination of natural and
management related processes. These
processes include:
Leaching
As water moves through the soil
profile, a slow but persistent acidifying effect
occurs from downward movement of cations
(bases) with the water. This is a very slow
acidifying process that takes hundreds of years
to have a significant pH change. Obviously, the
impact would be greater in the higher rainfall
areas compared to drier climates with little
leaching. This long-term effect in the United
States can be visualized as you think about
more acid soils existing in the east compared to
the west. Acid rainfall also can contributes to
the acidification effect, but the concentration of
acid in acid rainfall is relatively low and on a
short-term basis is not a major impact on the acidity of soil.