This happens because resources and opportunities to earn money in rural areas are very limited, so people try to move to cities to improve their lives.
I often see this because most young people who graduate from high school will leave their hometowns to find work in the city.
There's hope that moving to the city might be more fulfilling, as there are more opportunities to change their fate, although there's no guarantee of greater success.
Several friends my age migrated to the city after completing their education and now have much better jobs. This is the reason most people decide to move to the city rather than stay in the village.