Not that I’m annoyed or anything, but just… the reduction wasn’t arbitrary at all. If you said katakana was arbitrary, I might agree, because I have no idea how the fragments were chosen, but hiragana are simplified 草書 aka Grass/Running Script. They’re a calligraphic form. Here’s a video I found about it: Links between Kana and Kanji
Now, I agree that handwritten stuff is often kinda hard to decipher (my pet peeve was the fact that one mangaka’s あ’s kept looking like… お’s, I think?), but that’s partly because we’re not used to how Japanese people write. I follow the rules I learnt from Chinese calligraphy, so I’m not at all used to kana that have no ‘texture’ e.g. no slightly wavy finishing touches at the ends of horizontal strokes, which is nothing like the smooth, unfussy writing of mangaka’s and many other Japanese people who upload handwritten stuff online. I think it’ll get easier with time. Anyhow though, the reason the examples you raised are things Japanese people can tell apart fairly easily… is probably because of stroke order and direction? When written on paper/in calligraphic form, you’ll notice which ends of the strokes are thinner, which will clearly tell you which is which.
Yup, one of the quirks of Japanese (and Chinese!) I wonder if Korean has them too… basically though, they’re kind of like the ‘proper terms’ for groups in English, like a ‘peck’ of peppers or a ‘gaggle’ of geese, just that they’re everywhere in Japanese.
I can’t really think of any other ‘why, Japanese, why!!!’ moments… except perhaps finding out that the 〜ている form is both ‘present continuous’ (i.e. ‘to be doing’) and ‘present state’ (something like ‘to have done’, because the action happened in the past, but the state remains). I don’t remember being taught that by my textbook, and I needed to learn it myself later on.