Rents do not have to rise. They have been rising in most cities because people are moving to cities. Rents dropped in Detroit when a lot of people left. (Rents also rise with inflation.)
I do wonder about the data. It compares the median rental to the median house purchase. And people usually rent smaller places. So, renting is usually cheaper because of that. I don't know that it is an apples-to-apples comparison.