They have a lot of brain and nose real estate dedicated to processing such smells for useful information and also stronger stomach acid and enzymes in the digestive system to make ingesting poop much less threatening for a typical dog vs. the average person. So they typically benefit from such investigations whereas people would get not much info from such behaviors but would be way more likely to get some awful disease. Better for us to have a body that says “uhhh don’t do that” whereas they have instincts that tell them to go right on ahead.