There are many reasons why people do not notice this, or why, if they do notice it, that they do not act on the realization, but the interests of the potential children cannot be among them, as I shall argue.
这句话精简一下就是:There are reasons why,(插入语), that they do not......。这里的that确实是多余的,但是考虑到这个插入语的存在,这个母语使用者应该是为了让自己说完插入语之后保证自己的思路是连贯的,所以用了that这个无实意的引导词来缓冲自己的思维,就是纯个人偶然化的口语行为。类比成中文,就像我们紧张时一直说那个,那个一样
why that 被隔开了 why that they do not act ... =why they do not act.... =that they do not act.... 相同句型有 莎士比亚戏剧『亨利五世』第五幕第二场 If I demandbefore this royal view Whatrubor what impediment there is Why that the naked, poor, and mangled peace, Dearnurseofarts,plenties, and joyful births, Should not in this best garden of the world, Our fertile France,put upher lovelyvisage?