Because they're busy doing the rest of their job? They don't have enough time for it, nor is it a good use of their time.

That's like asking why the CEO of a 20-person startup isn't reading every line of code for bugs. It's not the best use of their valuable time.

It's their responsibility.

It is, therefore, absolutely a good use of their time.

That's not how any management position works, which is what an editor is.

You're responsible for verifying that the output looks sane and that processes are good and appear to be followed.

You can't double-check every tiny detail. That's absurd. At some point, you simply have to rely on the word of your employees, and fire them and do damage control if it turns out they're not following procedures but claiming they are.

You seem to be asking for an impossible level of quality control, with the budgets available.