Isn't it common in English, too? Just to mention "vague", "awe", "sigh". There are many others. I see those words and pronunciation is far from obvious to me.
It is, but in Japanese it's on a whole other level. One key thing a lot of people don't really account for is the fact that native speakers of a language are already fluent in their language before they learn to read, and in many cases (certainly not all) have the added benefit of having heard the word before and the task is just to create a mapping between the sound and it's visual representation (for words already in their vocabulary). When you're a non-native speaker, you don't have this advantage and the difference it makes can be quite large. So, it's natural for anyone non-native speaker to struggle with this. The difference is in frequency, severity and duration. You can get a pretty good hint at how much insanely harder it is for non-native Japanese speakers learning to read Japanese by comparing how much native speakers both kids and adults struggle to read unfamiliar words. I can't really ever recall struggling to read unfamiliar words much in English, and I compare that with my wife who is Japanese and how many times we have played a guessing game at how to read an unfamiliar Japanese word in a book she was reading. It took me a long time to realise that even for Japanese people there's just this level of insurmountableness to it that is part and parcel of how the orthographic system is structured. Going back to how kids learn to read unfamiliar words, books in Japanese have furigana in them which are a phonetic guide on how to read words that the author assumes the intended audience has a high chance f not being able to read. Even general purpose books aimed at adults have quite a lot of this. The younger the intended audience, the more furigana tends to be present as it's simply not possible to read without it when you're much younger. What you often see, though not always, is for a difficult word to have furigana the first time it appears and then subsequent appearances of the word don't have it. This is one way you learn to read more over time. For certain words, no matter how many times it appears, if it's obscure enough it may always appear with furigana.
The existence of furigana alone is a huge admission that Japanese a somewhat unreadable language that has somehow or other been made to work through bolting it on. So, you can imagine that if an orthographic system comes with permanent training wheels for native speakers, then non-native speakers are going to have a pretty brutal time with it.
This was the point I made at the top of this thread: that Japanese manages to combine the complexity of written Chinese with the inconsistency of English spelling. Does any other language expect quite so much of its users?
The existence of furigana alone is a huge admission that Japanese a somewhat unreadable language that has somehow or other been made to work through bolting it on. So, you can imagine that if an orthographic system comes with permanent training wheels for native speakers, then non-native speakers are going to have a pretty brutal time with it.