Because the “something” is often people’s livelihoods and reverting policy doesn’t always undo the damage it causes.

With software, nothing.

With human systems? The impact of your change is not always readily apparent, easily traceable, or based in logic.

Particularly, because there is always going to be lag between change and outcome as the humans adapt their behaviour over human time scales, not computer time scales.

Implementing any political changes requires support from other stakeholders. After a politician breaks things a few times that support evaporates, regardless of successes in other areas.