A unit acquired in such a way would likely have very low loyalty, and the side doing it would be committing to paying this low-loyalty unit's upkeep indefinitely, without necessarily having regularly occurring use for it to get their money's worth. Also, the casters that are in such a situation are likely among the lowest level and least skilled/useful, because if they weren't they'd likely be finding enough jobs to easily support themselves. So, a hypothetical side recruiting permanent members that way would be putting a lot of money in recurring payments to acquire incompetent likely turncoats without knowing or planning for their specialties in advance.
But the real reason is probably more "It Just Isn't Done!"
That wouldn't stop Parson, of course, but I'd guess no one's mentioned anything about it to him so he doesn't know it's an option.