Why do people say aviation regulations are written in blood?
No, that's all it means. Aviation regulations tend to be put in place in response to accident reports, where a plane crashed or almost crashed for lack of the regulations. So, if you're confused why a stupid-seeming regulation exists, there's a good chance that people died because of the thing the regulation changed.

Obviously, not all regulations are like that; for instance, noise limits are based on quality of life for those near airports, not crashes. However, a good number of aviation rules came into effect as recommendations following a crash report.