It drains and dries quickly. Nutrients also tend to leach out before plants use them. Humus worked into sandy soil acts like a sponge, absorbing moisture and holding nutrients. Clay soils are thick with small particles and tiny air spaces. Clay particles stick together, causing water to fill up air spaces.
Plant roots rot and suffocate from too much water and too little air.
Add humus to prevent the small particles from binding so tightly. Humus causes clay to collect into larger clumps creating larger spaces that allow water to drain and air pockets to form. Remember, soil can become sterile over time. Hot weather reduces humus content, topsoil washes away if bare spots exist and micro-organisms become depleted. Fertilizer provides nutrients to plants, but it's not the long-term solution.