Is weathering solely responsible for soil formation? If not, why?

No. Soil formation is responsible for the number of factors. Weathering is the first and prime important for soil formation. Then the colonisation of bacteria, lichen, mosses, and sheltering of other organisms in the soil help in formation of humus. Then other grasses, plants, twigs get mixed to it and make the soil fertile. As air gets into it, the soil gets nourished.