On a per-acre basis, young, fast growing plants produce more oxygen (and consume more carbon dioxide) than older, slow-growing plants. Agricultural crops, therefore, are the best carbon sinks available. They are far more productive in this respect than tropical rainforests.
Why aren't we planting more of them? Because the same people who insist that global warming is a problem, consider agriculture to be one of the biggest environmental polluters out there. They are also against nuclear power, even though it doesn't produce a single molecule of carbon dioxide.
So the real question is, why are the people who claim to care about global warming fighting against two things that would go a long way toward reducing the amount of carbon dioxide in the atmosphere? And the answer is that they don't really care about global warming. What they really care about is destroying capitalism.