Why don't all elements have half lives?

Not all elements have half-lives because half-lives are characteristic of unstable isotopes that undergo radioactive decay. Stable isotopes, which do not undergo such decay, do not possess a measurable half-life. Additionally, elements that are entirely stable, such as carbon-12 or oxygen-16, have no tendency to change into other elements, thus lacking half-lives. In essence, half-lives are relevant only to those isotopes that are unstable and will eventually transform into different elements or isotopes over time.