The issue is that trees release that CO2 back into the atmosphere when they eventually die and decompose. Only expanding the coverage of living forest actually

The issue is that trees release that CO2 back into the atmosphere when they eventually die and decompose. Only expanding the coverage of living forest actually captures carbon, and it's a one time thing. As a reference, an acre of forest in Maine stores about 400 tons of CO2, putting away about 8 tons per year until it reaches maturity. To expand forest coverage you're either taking land that was previously deforested for some reason (for example agriculture), in which case you're competing with that alternative land use, or you're planting trees in an area that never had forest, almost certainly because the location is not naturally conducive to forest, meaning you need to expend substantial resources to make it (possibly literally) take root. You can get more out of it by burying trees after they die such that they can't decompose, but this is a substantial labor investment. With weathering, the carbon is permanently locked away and as long as there is more rock to weather you can keep capturing more and more carbon. We already produce billions of tons of waste rock each year as a byproduct of mining. The bottleneck is entirely the extraction of desired resources from that rock. If we didn't care about that, it's really easy to just blow up massive quantities of rock to dramatically increase the exposed surface area.