Why can humans not rely on the transport method used in small organisms

This is because human have a much larger surface area. Small organisms have a small surface area and thus simple diffusion meets their transport requirements. As the size of the organisms increases, its surface area to volume ratio decreases. This means it would take days for molecules to diffuse through a large animal, so humans have a circulatory system with a pump to transport materials quickly around their bodies. Therefore due to their large size, humans require a specialized transport system.