The problem isn't lack of source. Most GPL programs that have appeared on the App Store have included a way to get the source satisfactory for GPL purposes.
The problem is with the terms of use of the store. When an end user tries to use the store, the end user has to agree to certain terms, including that they will not reverse engineer anything they download, they will not copy it, and other terms of that kind.
When a developer offers his program through the store, he provides a copy to Apple. When an end user purchases the program, Apple makes a copy and provides it to the user. Hence, from a copyright point of view, Apple is making and distributing copies of the program, and so Apple needs permission of the copyright owner.
If the program is GPL, Apple has to obey GPL to get that permission. GPL prohibits imposing additional restrictions on the GPL code you distribute. Apple's end user terms of service are additional terms.
Hence, it is not possible to distribute GPL code through the Apple store unless you grant Apple an exception allowing them to impose their additional terms. If it is the copyright owner who is making the app, that is not a problem. However, if they use anyone else's GPL code, and that third party coder has not granted Apple an exception, then the app cannot be distributed via the store.
The problem is with the terms of use of the store. When an end user tries to use the store, the end user has to agree to certain terms, including that they will not reverse engineer anything they download, they will not copy it, and other terms of that kind.
When a developer offers his program through the store, he provides a copy to Apple. When an end user purchases the program, Apple makes a copy and provides it to the user. Hence, from a copyright point of view, Apple is making and distributing copies of the program, and so Apple needs permission of the copyright owner.
If the program is GPL, Apple has to obey GPL to get that permission. GPL prohibits imposing additional restrictions on the GPL code you distribute. Apple's end user terms of service are additional terms.
Hence, it is not possible to distribute GPL code through the Apple store unless you grant Apple an exception allowing them to impose their additional terms. If it is the copyright owner who is making the app, that is not a problem. However, if they use anyone else's GPL code, and that third party coder has not granted Apple an exception, then the app cannot be distributed via the store.