Why is rain not naturally acidic?

Rain is naturally acidic.

As water droplets, when falling through the atmosphere, they dissolve small amounts of carbon dioxide from the atmosphere, to form a very weak acid(Carbonic Acid ; H2CO3).

This acid gives rain water a very slightly acidic pH ~ 6.5

Humans and other animal life create carbon dioxide by exhalation. This is a natural process , and its absorption by rain water is also a natural process.

However, because of industrial activity, more carbon dioxide and sulphur dioxide are released into the atmosphere, which is also dissolved into rain water, creating 'acid rain' in the process ; pH ~6 , slightly more acidic than natural rain water.