I was wondering why fruits, vegetables, animals and plants, are written in katakana? Often, there exist kanjis for those, so why aren't they written with those kanjis instead? For example:
This depends on the type of the words.
As for easy and common words, such as 桜, 犬, 蚊, they are usually written in kanji. These are written in katakana only in biological contexts. 常用漢字表 generally tells us what is considered easy and standard in modern Japanese. If you wrote "東京はサクラがきれいです" or "イヌを飼いたいと思う", that would look unnatural.
Relatively difficult words whose kanji are not listed in 常用漢字表, such as 薔薇, 豹, 蜻蛉, are usually written in katakana or hiragana in newspapers and official documents. People can generally read these kanji, but don't remember how to write them with confidence, because they are not something we learn at school. Using kanji for those words (with the aid of IME) is generally safe, but that may look too stiff or literary. Whether to use katakana or hiragana is up to the writer's taste. Katakana may look a bit technical, but it does stand out in the sentence, which will help us read smoothly.
There are hundreds of plants/animal names whose kanji are simply too difficult. I think majority of the Japanese population do not know (and do not want to know) how to read 紫萁, 臘虎 and 蟷螂. We have no choice but to use either hiragana or katakana in these cases.
Loanwords such as バナナ, チンパンジー, ピラニア must almost always be written in katakana. This is not surprising, is it? There may be 当て字 or corresponding Chinese names (香蕉, 黒猩々, ...) but they are only for kanji maniacs.
If explained within the framework of Tinbergen's four questions: (don't take it too seriously)
The proximate explanation is, because it's a convention in the biological society. In academic field, every creature's name is written in katakana when it refers to a equivalent of a scientific name. It once was even required by law (though it didn't state explicitly to use katakana). Today, the practice is getting more and more popular, which I think is because we do it so extensively in the science class that even people unrelated to biology prefer katakana for any plant or animal name.
The ultimate explanation is, it stands out in the text. The original reason why biologists started to write them in kana, I guess, was they wanted to distinguish them from ordinary words, as Japanese writing doesn't have italics or Capital Letters. Interestingly, in the pre-WWII ages academic documents were commonly written in a mixture of kanji and katakana, so they used hiragana to render those terms. Nowadays we use hiragana for standard orthography, and katakana for foreign words. Since many plants and animals are innately written in katakana, it may feel consistent to write all of them in the same way.
Note that not every situation is appropriate for katakana. Vegetables, meats and fishes in the store (as well as woods) are less likely to be seen written in katakana. Generic words (fish, bird, mammal, or bear when you're talking about many bear species) are often left in hiragana or kanji. Advertisers prefer kanji, especially when they want the advertisement to look classy.
I think it has more to do with style.
Legibility - There are times where it is easier to read or glance at when written in hiragana/katakana.
Kanji difficulty - For example, 豌豆{えんどう} is a very difficult kanji to begin with. Simplifying to hiragana/katakana would make it easier to understand.
Aesthetics - Perhaps it lends itself better to the presentation of the document.
Word origin - For example, pumpkin is written in kanji as 南瓜{かぼちゃ}. It uses 熟字訓 by applying an existing Japanese pronunciation to an existing set of kanji of Chinese origin, but may not always be intuitive or recognizable. This is opposed to, for example, 大根{だいこん} where almost everyone recognizes this because it is read naturally and the kanji matches the pronunciation of the word.
