Aircraft must be the antithesis to Agile work and continuous iteration. It's a completely different mindset to anything I've done. Imagine it: 'I pushed a new flight yoke to 737 v2.199. It crashed so I've fixed the bug in v2.200. '
Also, even the more stable and mature aircraft designs get relatively frequent software updates, and there is a lot of software in aircraft these days. The interesting part is where the faster iteration process of software development meets the slower process of aircraft development. The software developers are constantly iterating just like you would expect them to, and then every so often spin off a version for release on the side. That release version goes through extensive testing (mostly regression testing to make sure nothing that used to work is now broken) before it ever makes it onto an aircraft. Usually a few fixes are required as well. While all of that is happening, there are still people working on the next version.