According to The Line Becomes a River, why do migrants no longer cross the border near cities?

The Line Becomes a River by Francisco Cantu explores life at the border of the United States and Mexico. With border security and immigration being highly charged issues in the modern political era, this book shows what it's like to be both a border patrol agent and a migrant trying to find a better life in the United States.
Many people illegally cross the border on a daily basis, but the trend has shifted. In the past, people would cross near cities so they could quickly enter a city and find shelter. Now, however, with heightened border security, cities are heavily policed and monitored, making it more difficult to cross the border near those locations. In between cities, where populations are sparse, there are greater swathes of fencing that is not monitored, and there are areas with weaknesses in the border, such as holes in the fence or tunnels underneath. This makes it easier to cross the border and affords the migrants ample time to pass through and get to shelter before they are seen by a border patrol agent.