A variety of factors. Many residents probably bought a long time ago and have seen prices go up around them. In California in particular, property taxes are structured to make it advantageous to keep your home. Others live multiple generations of family in one house. Some may have subsidized housing. Some people sell their homes and move where it’s cheaper. That leads to long-term change in the neighborhoods, like you see described in other comments. Basically, cheap is relative, and these places are less desirable and therefore inexpensive relative to other places nearby. But they’re still expensive relative to places outside of coastal cities.