A large number of these people are not just from a poor country. Some are fleeing for their lives.

Also a reality for all of them that is completely ignored, (probably because it would require volumes of books and research to fully grasp) is that there is no country in Central America that is completely sovereign in the sense that people raised in the states think of it. Their economies, (ie job opportunities, cost of food, availability of health care etc) are by and large a subset of what has been set up for them (IMF, trade agreements written by multinational corp, world bank, mining and logging rights...the list is long). Their governments have learned that they not only must be friendly, they must agree to whatever conditions that the US demands, so they agree. Any dissent that becomes organized will be dealt with; often with lethal force outside an official govt role.
(I have a friend from Guatemala whose mother bought he and his brother here some years ago in just such a situation...his father had been murdered and she was informed they were next when they got around to it)

It is complicated beyond my bits of knowledge and no one is saying making open borders. But it is ignorant and dishonest to pretend that the USA is not part of the reasons for why people are trying to migrate.