There is a very good reason: the maintainers want to make the change. They don't even need a good technical need to do it, but usually there is at least something that drives the change.
It’s something the maintainer made and licensed in such a way that you can use it for free. The special status concerning “his” code is that it generally is in fact “his” (or her) property.
If you don’t like it you’re free in most cases to fork it and suit yourself.
Either way, it still doesn’t change the fact that there are going to be upstream bugs and vulnerabilities that remain unpatched if you’re trying to keep something frozen for 15 years.
An open source maintainer is a person volunteering their time and resources to offer something usually free of charge. As such, they decide what usecases they want to support and for how long. You can't expect someone to do work for free to support you.
There seem to be this sentiment that maintainers owe the users nothing, but also the opposite is true.
There is no reason to give the maintainer some special status concerning "his" code.