If the owner of the code is publishing my the app, there is no problem as they just own the code and aren’t using it under any license and aren’t bound by the GPL. Similarly an owner of GPL code can relicense it for use on the App Store. Not ideal, but it can be workable.
But that's not the case most of the time. Many apps aren't just one man operations, they pull in contributions from other authors and if it is GPL in any way, it becomes a mess to change that after the fact.
