Why would cast iron vent stack corrode? Rain water?

[Ednamae]

Thanks in advance. The cast iron vent stack (apparently coming off our 1965 main drain line in the slab with nothing draining into the stack other than perhaps rain water from the roof) is completely rotted through at the bottom in multiple cracks and I assume leaking sewer air into the house. The pipe is slightly off kilter...tilted, but still, why would it be rotted through? Stress cracks from the 50 year old crooked install? Or does it indicate some other problem that I am not imagining? Water backing up into the vent stack? We just bought the place so no idea. Multiple things seller did not disclose....like sewer odor I am assuming he smelled. Stack is cast iron for a few feet then 2 90 degree bends in galvanized to the roof, all original.

 

[hj]

It deteriorated because the air inside it is warm and humid, and there is no water running down the pipe to "wash it". It is a very common problem with old "vent" lines.