One can only get Karet (being cut off) once. Why should someone who violated a law with a punishment of Karet refrain from committing it again? For example if one ate on Yom Kipper why can’t that person eat chametz on Passover? Or if one had sex during menstration why not do it again? G-d can give out this punishment only once. Also why are there many genarations of irriligious jews shouldn’t they have died out from Karet?
If one truly believes that his (or her) actions have made him liable to karet, he should certainly want to repent in a way which will atone for the sin and mitigate the punishment. HaShem has given us some indication of the workings of Divine reward and punishment, but (as Rabbi Moshe Chayim Luzzato has pointed out in Derech HaShem) it is impossible for a mortal man to fathom the exact working of HaShem's system of governing the world.