Questions starting with 'why' are rarely useful in English! The answer is almost always 'because that's how it is'! But - you don't have to say 'a human being'. You can also say 'a human'. Note that 'I am human' is something quite different. That is using 'human' as an adjective, rather than as a noun. That's a subtle difference, but it is a difference. I suspect that using the word 'being' is either for emphasis, or possibly to avoid ambiguity.

Your sentence also gives good insight into why people add being onto the end of human - often to emphasise it, or just because they have been taught it. Your sentence- I'm always was interested... Would normally be written - I'm interested in why Adding the 'always' and 'was' gives more meaning to the sentence for the reader, to be grammatically correct it would be better to swap the two words - I was always interested... OR remove a word I'm always interested... Using was also implies that you may no longer be interested as it creates past tense.