Agile is not sufficient for regulated industries.
For example, each commercial aircraft in the US has their own set of engineering designs specifically for that aircraft. Every single nut, bolt, and rivet is documented and signed off my multiple engineers - materials, electrical, mechanical, stress, etc. In the event of a plane crash, the FAA swoops in, grabs up all the pieces and reconstructs everything to determine the cause of the crash and they review all engineering drawings and documents. If the cause of the crash was due to the design of the aircraft - a lot of engineers are going to lose their license to practice engineering. Now, think about the "auto-pilot" software or other control software on the plane. If the plane crashes, do you think that the FAA will accept index cards as an acceptable substitute for documented design and specification?
How about the anti-lock breaks and traction control software in your car?
Or the software that is used to control medical equipment? I suppose you are not familiar with the programming mistake in the software of a nuclear medicine machine that exposed a patient to 20,000 times the expected dose of radiation and killed them.