Look around. Almost everything is made of carbon. This is because carbon is an essential element in every organic compound. When you plant a tree, the structure of the wood, the bark and leaves is made mostly of carbon. So instead of floating around in the atmosphere, it's "sequestered," or trapped, in the tree's biomass.
According to the U.S. Department of Agriculture, an acre of trees sequesters (very roughly) 361 tons of carbon dioxide over 100 years. According to the radio show Car Talk, an average car emits a little more than 6 tons of carbon dioxide every year. So to offset one car's emissions for one year, you'd have to plant roughly 2 acres of trees (and continue doing so every year).
So, while planting biomass is a nice thing you can do today, on a population scale, the math of trees versus carbon just doesn't pan out.
Instead, scientists are looking at other places to stick carbon -- where the sun don't shine -- specifically in oceans, peat bogs and underground.