Why is there no long あ and い in on'yomi?

for りい, it seems to exist:

  • 爬龍 or 爬竜

also, these have しい reading variant:

  • 弑逆
  • 詩歌 (learnt in WaniKani)

long う could be in readings with unvoiced consonants, to indicate that the vowel should be pronounced. otherwise it can be omitted, like it happens with any 不… readings.

These are Okinawan, not standard Japanese.

Weblio says the しい reading is idiomatic.

Jisho rubies this one as し+いか, though I’m not entirely sure how accurate that is.

Speak of the devil (and he […eventually] comes).

I took this 7 days ago and thought of this thread.

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