When buying a home many inspectors that I came across inspect almost everything except camera inspect main sewer lines.  Visually in my area Metro Detroit many of the homes in the market were built back in the 1930's and 50's. So the main sewer lines were made of cast iron and in some cases clay material, which can allow roots to intersect the pipe and cause major problems. 

With that said I bought 4 homes and of course 2 of them needed work on both main sewer lines. It sucks. I rehabbed both units. and literally 1 year later the drains started to back up. on 1 home I was stuck paying 5,200. I would have lowered my offer if I had known there was something wrong with the main sewer line  in the beginning when I bought it.

People are still buying old homes that have wooden pipes. To replace that would probably cost an arm and a leg.