Comment by that_ant_laney

> What do you mean Mandarin is simpler in this regard?

Just to add context to a sibling comment, Japan's first "writing system" was literally just Chinese.

I don't mean Chinese characters, I mean that if you wanted to write something down, you had to communicate in written Chinese. Over time this written Chinese accumulated more and more transformations bringing it in alignment with spoken Japanese until we get what we see today. However, this means that, to a first approximation, modern Japanese is some amalgamation of Old Chinese and Middle Japanese.

Actually, use of Chinese co-existed alongside the whole transformation process, so we actually see this funky mix of Early and Middle Japanese with Wu, Han, and Song Chinese. Character readings varied by region and time period, and so the the reading of a compound kanji term in Japanese mostly reflects the time period when that word was imported. This is why a single kanji ends up having multiple readings. Later, people began backporting individual characters onto native Japanese words, giving yet another reading.

The character 行 is a particularly illustrative example: 行脚 (an-gya), 行動 (kou-dou), 行事 (gyo-ji). The first reading "an" comes from 7th century Chinsese or so, "kou" comes a bit later from the Han dynasty, and "gyo" even later from Song. Then we have the backports: 行く末 (yu-ku-sue), 行く (i-ku), 行う (okona-u). The first "yu" reading is from Middle Japanese, "i" from Modern Japanese, and "okona" from I have no clue when. That's six different readings for 行 alone!

Oh, and then there are "poetic" readings that are specific to usage in people's names: 弘行 (hiro-yuki) etc. Granted, these are often quite evocative of the above readings or that of synonym characters.

The historical introduction process also explains why older readings tend to be more obscure, 1) they had less time to accumulate usage, and 2) they tend to be specific to Buddhist and administrative themes.

Note: The above is just what I've pieced together osmotically over the years, so I'm sure there are errors.

Hanzi as used in Chinese usually have exactly one reading. On the other hand, virtually all kanji in Japanese have several different pronunciations depending on context.