there are two ways to understand this: 1) kares means that if a person does a certain thing, the likely natural outcome is that they will cease to be part of klal yisrael. For ex., if a person marries a non-jewish woman, his children will not be Jewish. you can find such reasoning in many things that are “kares”. Some are more obvious than others
2) if a person does that particular thing, they have gone so far that they have lost their olam habah
Furthermore, those actions that warrant kares probably only do so when the aveirah is done knowinlgy and intentionally.