The way these people talk about the past makes it sound like people in the past were somehow perfectly stoic in the face of discomfort or disease, but it's extremely easy to find historical documents showing that this isn't the case. I think the reason people fall into this "noble savage" trap is because they (rightfully) can't imagine living without modern technologies like refridgerators and flushing toilets, and therefore (wrongfully) conclude that the people in the past were simply more rugged to be able to live without it.

Also, as you say, it's an easy way to explain away our problems, to say that people in the past were better and if they were here today then they wouldn't be complaining like we are.