Because any current S-ATA platter based HDD (almost all nowadays) are not as fast as S-ATA 3.0Gbps (current common interface) even provides, they hardly saturate S-ATA 1.5Gbps (approx. 150MB/s) with their maximum speed of 120MB/s. SSDs are providing faster speeds, but S-ATA 3.0Gbps and 6.0Gbps is sufficient for them too.