They are cheaper and they pass IPXX requirements on dust/water protection easily. But they seem to be good enough because customers, despite some complaints, keep buying devices with capacitive buttons.
Also, mechanical stuff eventually wears out - at best with good quality ones, the product becomes obsolete before they do. For instance potentiometers [1] used for volume control on stereos rust over time and eventually become unusable. So there's a durability argument too.
