Because it's a waste of time and for that reason will be ignored.
Googlebot and the robots for all other search engines always follow and (potentially) index links that they find on a page unless they are explicitly instructed NOT to do so, either with a tag like <meta name="robots" content="noindex, nofollow" /> or with an instruction in robots text NOT to index or follow links in specific folders or by adding rel="noopener" to the end of a specific link.