Take, for example, katakana such as ウェ, ティ, or ジェ. Many, many textbooks and other language sources, when showing a list of katakana, omit these.
In particular, a lot of Japanese language books show full-looking hiragana and katakana charts, and they'll mention the special "f-" ones (ファ, フィ, フェ, and フォ), but not breathe a word about most of the others
This is very, very common, but...why? These are not obscure, archaic devices. They are used in everyday life. The Japanese word for "Sweden" uses ウェ. The Nintendo Wii uses ウィ. "Jason", a common English name, is best transcribed as ジェイソン, which uses ジェ.
So since these special syllables are not unusual...why is the culture surrounding the Japanese language such that so many sources pretend they don't exist?