Because they are expensive and useful and if they have autonomy with goals that do not include self-preservation, they might end up destroying themselves in ways which are expensive and wasteful.

(Why would the sense of self-preservation not be calibrated to be exactly at the level to control costs without interfering with other interests of the owners? The same with the degree of autonomy and other aspects implicitly involved in the hypothetical, it wouldn’t, intentionally, but complex systems are hard to predict, so calibrating it exactly right will be hard.)