Why do anime characters not use the F word?

I’m a Japanese and I answer this question as a native speaker.

First of all, Japanese has little vocabulary of curse words unlike other languages, as others suggested.

In English, there are tremendous number of crotch-related insulting words like “fuck you”, “asshole”, “dickhead”, “motherfucker”. However , as far as I know, there are no such word in Japanese. Moreover, if you said 犯すぞ(fuck you), おまんこ野郎(you cunt) or so on, those would be weak as swear words. On the contrary, it can even sound funny because they are unnatural.

The strongest curse words used against your enemy I can think of is 殺すぞ(korosuzo, I kill you) and 死ねよ(shineyo, die). If you say this to a Japanese guy, he/she will be very disgusted and angry and there might be even a fight.

The most often used curse word is クソ(kuso, shit) in my opinion, which is said when you failed. For example, “クソ、またやっちまった”(Shit, I failed again). This is mainly said by men when they are talking with their friends or families, but not in front of their bosses.

Short swear words like ボケ(boke, idiot), クソ(kuso, shit), コラ(kora, an intimidating interjection) are often added to the end of swearing sentences.
For example, “何見とんじゃボケ”(What are you looking at, idiot).

Lastly, I show a derogatory conversation of two people extremely angry and quarreling.

“お前なんや！誰に何しとるのかわかっとんのか！” (Who are you? Do you understand what you are doing to who?)
“ああ？調子乗ってんじゃねえぞコラ！”(Huh? Stop getting carried away!)
“お前頭おかしいんか、おお？” (Are you a nut?)
“なめてんのかこの野郎！”(Are you taking the piss out of me?)
”何だテメエ、オラ！”(What are you?)
”おい何じゃボケ、やんのかコラ！”(What are you? Are you gonna fight with me?)
“殺すぞテメエ、クソが！”(I’m gonna kill you, you shit)
”やってみろやボケ！”(Try it, idiot)

As you see, there are no groin-related word. Japanese is a kind of a “clean” language. However, this is one of the reasons translators suffer from translating foreign derogatory words into Japanese because there are often no corresponding word.