Why some words that are extensions of words, are not explained with those original words mentioned/referenced in meaning description?

Solution 1:

One reason is that etymology is not always a reliable predictor of the current meaning or usage of a word. While it might make sense to think of 'iffy' as being based on a conditional (and therefore unreliable), the modern definition can include 'not very good' in modern English ("an iffy neighborhood"). This meaning is not quite the same as 'unreliable'. Already, the usage of the word has shifted a little away from the original.

While there are many cases where etymology can help memorize a word's meaning, on the whole, it's just not reliable enough as a strategy to make it a core principle of learning vocabulary. That is not to say that learners can't benefit from incorporating etymology into their studies. They certainly can, but you seem to be suggesting that it should be given more attention at policy level or a wider pedagogical level. I'm not convinced it's justified.