I’m not surprised. And i’m also not disagreeing with you, as you will see in a moment…
I’d say, for students of any given language to learn its pronunciation they need at least
- the ability to retrain their facial muscles
- the ability to hear the differences between native-like pronunciation and their own pronunciation
- the ability to discern intonation, pitch and tone, as required - this ability is likely related to what in another context is called “musicality”
These abilities develop in people through the course of their learning experience, but not to the same extent in all people: some end up being better at pronouncing the new sounds than others…
What helps, especially if one wants to learn a language in a rather short amount of time, is interaction with someone who explicitly teaches the pronunciation (this includes modeling for learners and correcting them).
The most important point we need to keep in mind in this context is that learning the pronunciation of words and phrases in a given language has in principle nothing to do with writing (witness how all children and also many adults anywhere in the world learn to speak one or more languages without any writing system being involved), but in practice, at least in the “developed countries”, most learners of a second/ foreign language are concurrently introduced to the sounds of the new language and the associated writing system, and thus it is often difficult to recognise and address the various problems that show up in the process (this simultaneity has advantages and disadvantages, but it seems very few language teachers/schools seem to care about any of this).
My experience is that people whose native language is written in a phonemic/phonetic system often have certain difficulties learning the pronunciation of a second language using the same writing system because of interference from the writing system they think they already know (and i had been addressing only those additional difficulties in my previous comments - if you read them that way you will see that we agree about the point you make, namely that otherwise existing difficulties in learning the pronunciation of a new language have nothing to do with any writing system).
Not surprisingly, native speakers of a language of which the associated writing system is either not phonemic/phonetic (e.g., Mandarin, Japanese to some extent) or which uses non-Latin characters (e.g., Korean, Japanese, Russian to some extent), encounter these additional difficulties when learning a third language - assuming here that the second and third language they learn both use the same writing system. That is to say, people who have learned the pronunciation of any one of English, French, German, Italian, Spanish, and who also know how to read in that language will only encounter the abovementioned additional difficulties when they learn another language with the same writing system: second language pronunciation tends to interfere significantly with third language pronunciation (a textbook case of this is Japan: in students in school learn Latin letters as one of the tools with which one can represent Japanese sounds (ro:maji), and after that they learn English. Not surprising, most Japanese speak - definitely read - English with a ro:maji accent!
In contrast, native speakers of Mandarin or Japanese who learn the respective other language don’t experience a comparable negative impact of the writing system they know on the new pronunciation they are trying to learn - i suspect, the reason is that (consciously or unconsciously) it is understood that what is written and how people pronounce their language it has no intrinsic connection.