I think street people have been exposed to a lot of viruses. Those that lived ended up with natural immunity, at least for some time, and were less likely to be harmed by it the next time. I remember at the start of this, fear was being ramped from every angle, and one of them was "street people are going to spread the virus like wildfire". But like you said, it didn't happen. Perhaps something similar going on in some third world areas, that were said to be ticking time bombs, but have reported almost no problems with Covid?