Why do textbooks and other sources pretend special katakana don't exist?

Question

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.

For example: https://upload.wikimedia.org/wikipedia/commons/0/05/Table_katakana.jpg

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?

Answer 1

ウェ, ティ and so on are collectively called extended katakana. As you can see in the link, the full list of extended katakana is fairly long, to the point where it's not suitable for beginners. Importantly, it's not for native Japanese words; most of them are used only when you have to represent foreign sounds accurately. I think it's a bit like é in English; although it's common in reality, it's for loanwords, and someone who learns English for the first time doesn't have to know it.