When you tell someone how a creative, unpredictable and intricate thing it is to create software, do they usually get it? I’d guess that usually no.
Sometimes maybe you get some «OKs», a few «Ahas» and other times maybe a few «I don’t think so». But at the end, most of the people who haven’t been in direct contact with this process at deep level, not necessarily coding, don’t have a clue of how a complex topic it is and the amount of variants that it has.
As Carl Sagan said:
We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology.
Could be that a good idea, before talking to non believers on Agile methodologies about why Agile is a better approach, make them understand how complex the process it is and why. Then, when they have understood the whole real problem, you could be able to talk about what we try to do to deal with all this complexity.