Because so far (as far as I know) we haven't found any sings of life from these very old star systems and because I went with the assumption that prior to the start of life on earth, the conditions in the universe would have been to bad for life to start development anywhere else. Also I formed my theory with no professional research or knowledge about the functioning of the universe and the formation of life and just based my theory on the odd YouTube video on the matter and my school education.