Apple doesn’t support it because GPLv3 requires that you are actually able to run the modified source code (not just see, change and distribute). While they probably could do that on macOS, macos and iOS share the same kernel and many common source trees. So by providing macOS software with GPLv3 code, some of that code could potentially be shipped with iOS which would land them in hot water because you are not able to just execute code on iOS. Even with a developer license, you couldn’t just recompile the kernel (or even Library) and replace it with the default one.
Edit:
Here is more information about that:
> Protecting Your Right to Tinker
> Tivoization is a dangerous attempt to curtail users' freedom: the right to modify your software will become meaningless if none of your computers let you do it. GPLv3 stops tivoization by requiring the distributor to provide you with whatever information or data is necessary to install modified software on the device. This may be as simple as a set of instructions, or it may include special data such as cryptographic keys or information about how to bypass an integrity check in the hardware. It will depend on how the hardware was designed—but no matter what information you need, you must be able to get it.
Source: https://www.gnu.org/licenses/quick-guide-gplv3.en.html
That means if there was GPLv3 code in iOS, Apple would have to send you their private keys to sign any source code so you can run it on iOS.