It's worth bearing in mind that the source is Japanese. All input methods for the Japanese language are a compromise. Inputting kanji on a physical keyboard is nowhere near as fluid as inputting Latin characters - you're constantly toggling between inputting kana and selecting the kanji options presented by the IME. The prediction and correction technologies of smartphone keyboards are a very good match for kanji and…
This is definitely a big factor. When you look at a Japanese computer keyboard [1] you might see lots of Japanese characters on the keys. Almost nobody actually uses these to type. Instead, if you want a か(ka) character then you type k,a on the familiar qwerty keys. These phonetic characters then get converted into the more complex Chinese-origin characters as you go along, allowing you to disambiguate homophones. Th…
How early did you learn to type? As an American born in 1980, I learned in first grade, and I don't remember it being a struggle. So maybe it's really important to learn early.
FWIW, I'm visually impaired, so looking at the keyboard wasn't a practical option.