How to make sense of うand く's Onbin 音便?


From what I have learned, 音便 arose for the sake of easier pronunciation and
they are changed from the ます form or 連用形 form.

However, I could not wrap my head around うand く. Why is that く is changed to い
but う does not? Take 書く and 買う for example, if 買いて is changed to 買って because
it's easier to pronounce, then why 書いて is acceptable? Why not 書って? I guess it
wouldn't be a big problem for communication as Japanese has so many homophones
already? (not sure)

Also, don't people find it confusing when they are used to say 買います, but then
かいて is actually 書いて but not 買いて？




I think [this](https://japanese.stackexchange.com/a/24044/45489) should
essentially answer you question. But anyway, the following is based on
[this](https://ja.wikipedia.org/wiki/%E9%9F%B3%E4%BE%BF).

What you seem to be missing is which 行 those verbs belong to.

The rules are as follows:

イ音便：標準語では、カ行、ガ行（すなわち「キ」「ギ」）において生じる。

  * 書く: か **き** て→か **い** て
  * 急ぐ: いそ **ぎ** て→いそ **い** で

促音便： 促音便はカ行、タ行、ラ行、ハ行の音（つまり「キ」 「チ」 「リ」 および 「ヒ」（現代の「イ」））に生じる現象

  * 買う: か **い** て→か **っ** て
  * 立つ: た **ち** て→た **っ** て
  * 降る: ふ **り** て→ふ **っ** て

