The answer is that no single system suits every country. In the complex discussion of income plans for the elderly, there are a few universal agreement and possibly fewer still that are not ambiguous. But most policymakers agree that many people, left to themselves, would not save enough for their retirement. So governments conclude that they have no choice but to get involved. Beyond that lies widespread debate over what's best and what's practical.
Nothing is simple.
Take the issue of how much people should be forced to save for retirement. Assume, for example, that some people save rationally while others save nothing. Forcing people to save would provide a welfare gain. The myopic will have something at the end. And if the mandated rate isn't too high, there's probably little effect on those who would have saved anyway.