"You need a warm jacket in the winter."
Probably because a) 必要 expresses a much stronger need, like an actual requirement or something you can't do without, and b) because it's a kanji compound. Duo is starting off with basic Japanese, in which you don't use many 'fancy' words like that.
Also, ー can only be used as a vowel extension in katakana; for hiragana you use a double vowel for 'a, i, u' sounds and add '-i' or '-u' for a long e/o sound respectively. E.g.: ひつよう