1) They become political. Rules are made (or not made) to appeal to voting blocks rather than by evaluating the science.
2) There is a strong tendency not to destroy that which exists. By any reasonable standard coal filed powerplants should not exist.
3) (Could be considered part of #2) There is a strong tendency to look at risks in isolation rather than in the marketplace. We should not be aiming to make industries as safe as practical, we should be aiming to make the outcome as safe as possible. These are very different things! The extreme example of this is electricity. Coal is ~10x as dangerous as oil which is ~10x as dangerous as natural gas which is ~10x as dangerous as nuclear. The risk to society is measured in deaths (or other harm) per terawatt-hour, not by whether any given generator is as safe as it reasonably can be.