ある is regular verb EXCEPT the negative of informal/normal form, the correct is ない and あらない doesn't exsit. The part あら disappeared. However, in archaic Japanese, it's said あらず (ず is for negative form) and in some dialects, they now say あらへん (あら + へん). In the negative of formal form is ありません. (あります --> ありません) I understand your argument but sometimes there are some you should accept. This is a part of LEARNING LANGUAGES. When we started learning English, we used to have the same sort of question in many cases. I hope this was helpful although only a bit.

Hi, Hyoshin! I red an interesting story about this, and here is what it's saying. So for example, the negative form of the verb ひらく is ひらかない, and it's antonym is しめる. And the meaning of the negative form and the antonym is different. Also, the negative form of the adjective おおきい is おおきくない, and it's antonym is ちいさい. And the same thing happens. And that's why these negative forms and antonyms both exist. However, when you look at ある, the negative form あらない and the antonym ない means exactly the same thing. So we started using just the shorter one ない. However, in my opinion, explaining the story like this to the learners is not so easy and might confuse them. I'm not sure if it's okay to say that the negative form of ある is ない, but I think there's no problem with remembering it like that. And the important thing in learning languages is to understand things as simply as possible. :)