It's a very important question, and the dynamics are very important.
I've spent several years working in irregular migration systems, as we call them, in north Africa, the Middle East and the Balkans, and now increasingly in Central America. The very basic fact of irregular migration—why markets exist for smuggling and trafficking—is that prohibition in the face of market demand creates illicit markets. People want to move and people need to move. As you mentioned, there are vast economic discrepancies between states, which act as the kind of push-and-pull factors generally.
When you force people to undertake irregular journeys with increasingly sophisticated and networked criminalized smuggling rings, very often they overlap with trafficking, and people can become trapped on their journeys. Women and girls can be pushed into situations of trafficking. Domestic labour is a major trend, but also forced sex work. It's a big issue that doesn't lend itself well to very simple solutions and policy statements, but the simple fact is the market dynamics that I alluded to.