reading 明日 – ashita why kanji for 日 – nichi dont have shita reading in any dictinary online?

Question

in word 明日 – ashita character 明 –has kunyomi reading (a) in every dictionary but 日 – dont have any shita reading or atleast i haven't found it in jisho or other online dictionary. same goes for other words like 今日 etc. and not only using 日 kanji thir are other words i came across where kanji reading is not mention in dictionary but they are pronounced like that. 下手 Heta like that下 dont have he reading Also 山田-yamada rice field dont have da reading in dictionary Are they special cases? is their a name for these kind of word or im just mistaken? Thanks.