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.

[1] https://en.wikipedia.org/wiki/Potentiometer

In the case of cars, isn't it simply that there is no other option on the market?