You misunderstand policy making. A lawmaker is not (and should not be) interested in providing a way of how to implement things, but how things should be, they are only providing the normative side.
The executive and judicative branch of the state then specifies how things should be implemented.
Most GDPR implementations that are so annoying are actually wrong.
I agree that this is how most legislation does work: dream up something that would be great and ignore the facts on the ground and live with a predictable shitshow of implementations.
To me it's a stretch to say legislation ought to work like this.
Many of the "outlier good ideas" in legislation are those that integrate the facts on the ground and consider e.g. implementation complexity.
Most GDPR implementations that are so annoying are actually wrong.