As a user, it doesn't matter to me why software updates have degraded like this. All that matters is that they have.
As a developer, I don't really understand. I've been working on large, complex software that targets multiple operating systems for years, and we don't have any such issues.
It sounds to me like a lot of companies are using development methodologies that are a bit broken...
I've long thought that the reason for this is rather different -- the industry really wants to go to continuous-release models (which I don't think is a good thing, but that's a separate issue), which make security-only releases a bit nonsensical.
Nobody does because it causes a combinatorial explosion in code branches, and in testing. This isn't just a problem with the higher level convenience tools like compilers and package managers, its a problem with the actual source code as formulated as it is today. We'd need an entirely different way of writing code in order to do that without a massive increase in programmer/testing hours. Personally I don't think it will ever realistically happen, but I'd love to be proven wrong.