Why does furigana often appear in the most unnecessary places in Japanese books?

Question

I'm been starting to try to read some Japanese novels recently, and one thing that struck me was that when furigana is used, it seemed to show up in the most unnecessary places.

For example, in the first few pages of Haruki Murakami The Wind-Up Bird Chronicle (ねじまき鳥クロニクル), furigana was given for

猫　【ねこ】、椅子【いす】、眺【なが】める、泥棒【どろぼう】、恰好【かっこう】、悪戯【いたずら】、and 蜘蛛【くも】,

all of which are common words that I'd guess are easily readable by most Japanese 10-year olds.

Maybe the last two are there since those words are usually written in kana, but the first two for example, make no sense at all.

Answer 1

I think the reasons are all various. According to the Association of Japanese Newspapers, 椅子 is “a word with complex kanji or an existing tradition of writing in hiragana,” that should prescriptively be supported by hiragana in writing. 猫, meanwhile, is a word that is known from childhood but its kanji is taught relatively late. 悪戯 is a very non-Jōyō usage, not listed among the allowed non-standard readings in the Jōyō, 蜘蛛 is literally made of non-Jōyō kanji. The novel was published in 1994-95, so 眺める and 泥棒 can be explained by the fact that 眺 and 泥 are still recent additions of 1981 and not everyone is guaranteed to know them. Only 格好 remains an outlier, but perhaps it is used in a context where rendaku is possible, to prevent がっこう? Also, apparently, in spoken Japanese it can be reduced to かっこ, and it could be to prevent that.

Answer 2

As @Chocolate states in the related link, I believe it has more to do with the age/grade the kanji are taught in school, rather than the commonality of them. So even though 猫 might be seen pretty often by young kids, several sources indicate that it is not taught until junior high school (beyond 6th grade).

Answer 3

It's not clear to me that furigana often appears in places where it isn't necessary. Possible explanations for the use in the case of your book are that the first character of isu used not to be in the Joyo kanji list until 2010, and there is also a rule that animal and plant names should be in katakana.

Answer 4

In addition to the other answers, I have also seen cases where, when dialogue is spoken by young children, sometimes the lines they say will use additional furigana (or just writing words out in kana instead of kanji) to give the feeling of the words being spoken by a young person (i.e. the included furigana/kana is made appropriate to the age of the speaking character, not necessarily the age of the expected reader, for stylistic effect)