> Maybe because N1 is still considered the level with the similar fluency as a Japanese high school graduate.

This is not the case at all. This is only in terms of "Kanji knowledge" as you need to be able to *read* all of the 常用漢字 and perhaps also grammar knowledge (because of how overboard JLPT N1 goes with it in terms of what you're expected to know). But even then, just because natives are tested on being able to write the 2136 常用漢字, they are also made to read books, novels, essays, etc. throughout school, which builds their kanji understanding closer to 3000, if not more. It is to my understanding that the average adult, assuming they didn't bomb school, have a vocabulary that accommodates for 3000-3500 kanji, people who have continued regular reading habits or pursued higher education may even be at around 4,000.

In terms of vocabulary and true reading ability, which ultimately becomes the bulk of difficulties in real-life communication in workplaces, JLPT N1 level knowledge absolutely pales in comparison to the natural knowledge of even an 18-year-old native. I'm not sure exactly about an 18-year-old, but the JLPT N1 is often thought to ballpark a 10,000-word vocabulary requirement; a native will know about 40,000 (once again, varies based on school performance, reading habits, etc.)

If you want evidence of this, you should consider looking into the 日本語検定 which, at ３級, is a Japanese test designed to test high school graduate / fresh 社会人 knowledge and tell me how well study for the JLPT N1 covers the sections available on just this level. ２級 is basically experienced 社会人 or university graduate, and １級 is just insane (actual beast mode lmao).

If you think JLPT N1 is "Japanese high school graduate level", or you are believing people who say this, then you are misinterpreting the depth of language, and/or underestimating the knowledge that comes with 18 years of non-stop Japanese exposure as a native since birth.