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？