Because then laws that people rely on become bargaining chips held hostage when the expiration date comes. And this creates horrible perverse incentives where the more obviously good and uncontroversial the law is, the more valuable it is as a hostage, so the more likely it is to disappear, which is a complete inversion of democracy. Like clean air? Well, give $MY_FACTION a billion billion dollars or it goes away.

This isn't a hypothetical either, you see this in American politics all the damn time with temporary laws and allocations.