Why do some people say that running is bad for you?
It can be actually bad for some people. I know some old runners who had knee surgery caused by running.
There will be probably no problem for the young, but as you grow older, the joints will deteriorate fast if stressed too much. The problem is HOW much before something bad happens. Some can take it, others may not.
Running is only popular for all ages and both sexes in developed countries, probably due to marketing and ego or herd mentality. In Asian countries, it is mostly the young who run. You will hardly see any retired individual running or using a treadmill. Most will be walking or cycling or swimming.