[simiones]

I think it's more that Japanese speakers just don't have those types of sounds in their phonetic repertoire. Some may be able to pronounce them, but most will not (and may not even notice the difference).

Every person has a certain limited set of consonants, vowels, diphtongs, triphtongs, tones, and even syllables that they are able to recognize and reproduce. This is something you can train to recognize more, but you will probably never be able to pronounce or even distinguish the totality of all those used in all languages, even just the living languages on Earth.

Even if you did, there is an added complication that some languages actually used multiple sounds interchangeably, and explicitly distinguishing them may actually confuse you. For example, most European languages recognize various consonants as the same "R" sound, even though they are vastly different (French R is a back of the throat trill, Italian R is a trill near the palate, and English R is articulated next to the palate without any trill). If you come from a language where these are distinct sounds, you may have trouble understanding that two people who use different R sounds are pronouncing the same word.