They come naturally because of reduplication and, as you mentioned, frequency of similar vowel sounds. However, writers and poets take advantage of them to create rhythm and rhyme.
I know that song, I think Kamikazee did that on purpose. Compared to other bands, their compositions are filled with lots of i-i-i-i-i and a-a-a-a-a. They pronounce their words with strong vowels and got distinct accent too. :)