The argument is that the details matter. Slightly different breeds of animal, different weather conditions, or different bacterial ecosystems affect the resulting cheese. You can make something similar to these products elsewhere, but it won't be exactly the same.
Some products are famous and specific enough that customers have a right to be able to know when it's the real deal (and there are many such customers - otherwise these names wouldn't matter at all).
If you want to produce something similar, you can start a new brand with other local businesses (Australeta say), label it Feta-like until customers recognize and choose it because they like it. You can agree whatever rules you like between you for how it's made and what constitutes 'real' Australeta so you have a consistent quality product.
Some products are famous and specific enough that customers have a right to be able to know when it's the real deal (and there are many such customers - otherwise these names wouldn't matter at all).
If you want to produce something similar, you can start a new brand with other local businesses (Australeta say), label it Feta-like until customers recognize and choose it because they like it. You can agree whatever rules you like between you for how it's made and what constitutes 'real' Australeta so you have a consistent quality product.
You can even do that and register your own new protected name - see the recent split of Corpinnat from Cava as a recent real-world example: https://www.thedrinksbusiness.com/2019/02/nine-producers-bre...