Store clerks(mainly grocery), when you see people stealing in self check out how often do you care enough to say something and why?

If it's a pack of sweets or a bottle of water it's genuinely not worth kicking up a fuss about, plus you could be wrong about them stealing which makes the whole situation awkward. In the case of potentially dangerous items (knives, razor blades, baby formula, etc) or specific valuable items (like alcohol, cheese, large meats) I may tell the security guard to confirm my suspicions and take needed action or bring it up to a manager. But for the most part it's not worth it to intervene or 'snitch', for my own safety or potential misjudgement.