One of the issues is that planting a tree is a slow form of sequestration. An individual tree may sequester a lot of carbon over its lifetime (measured in decades) as it grows, but it doesn't (and can't) immediately offset things as a seed or a sapling. Some carbon offset schemes attempt to amortize for this with accounting tricks, but for the most part when people are buying things like carbon offsets they are offsetting a pound of CO2 (or equivalent) emitted today with the hope that maybe a tree planted today will maybe sequester that pound over say the next ten years. (If it survives, if it is in a forest that is well maintained and hopefully not in a region prone to wildfires, if that forest is protected as an entity and not clear cut, and so forth.)