The publishing industry outsources much of its activity. Over the last forty years or so, the in-house roles have become fewer and the outsourcing more plentiful. The current eBook boom has changed the dynamics in an interesting way.
The tasks and roles that have been outsourced before the current revolution have been well understood. We have clients who have outsourced the entire production process and others who outsource just typesetting. Proofreaders and copy editors are not generally in-house roles these days. However, an important shared feature of all these roles and tasks is that publishers understand them. There is no inherent problem with outsourcing proofreading to a professional proofreader (nowadays, almost certainly a freelancer) because the publisher understands what a proofreader does and is able to determine that the work has been done well (or not).
Now, many publishers are outsourcing their digital production to third party suppliers, all claiming mastery of the subject. In the vast majority of cases, the eBook is created from the the print PDF file (although creation from InDesign and other application file formats is becoming more common). There is a problem with this process. Many of the publishers following this path (I know of some very honourable exceptions to this — Random House UK come to mind) have absolutely no knowledge of the technicalities of the eBook conversion processes or of the resulting eBook.
If the publisher does not know how to do the task themselves (as part of their corporate knowledge), they have no way to be determine the quality of the product they are selling.
There is no way that an editor would allow a print book to be put on shelves before they had seen a physical copy and, at the least, skimmed through it. The task of making sure that an eBooks is “good” is much more complex yet we see publishers assuming that their supplier is an expert all the time. It is just as necessary for the editor to be sure that the eBook is of the required quality (a difficult term in itself) and much harder to do it.
If you want to sell quality, you need to understand what you are selling. There is no reason why publishers should not outsource digital production but they really shouldn't do it until they understand how to do it themselves.