It doesn't help the Southern US if Central America has higher rainfall.
i.e. there is no guarantee that the rain falls where it's needed for farming. Much the opposite, as it turns out.
Agriculture is not just a way to make food, but a source of livelihood for millions of people. Most non-developed countries depend on agriculture to provide work for their people. This would affect them the most, putting most of them out of work.
There's also the simple factor that countries want to be self-sufficient in their food production.
To adapt a popular saying: All rain is local.
That is, rain is more or less useful depending on where it falls. More to rainforests, mountains and the seas would not be useful.
This is part of why people say "climate change" instead of "global warming", because while the world warms on average local temperatures depend on disruptable weather patterns.
Wait, what? A global water shortage is one problem that global warming would actually help fix. There might be local droughts, but it's meteorologically obvious that higher temperatures = higher evaporation = higher global rainfall. A warm ocean pumps out more water vapor, and warm air can hold more moisture (which is why Antarctica is so dry).