Are you confused, or do you not understand why they choose the path they do?
For obvious reasons it's easier to make landfall in Central/South America and then travel overland to the US southern border (even Cuban immigrants do this now). If you come up from South America you have to cross the Darien gap (Southern Panama/Northern Colombia). Considered to be the most dangerous place on earth because there are no roads, no law, cartels, native tribes, extreme terrain, and unpredictable weather as it's the rainiest place on earth.