The thing is that programmers largely
are interchangeable. Of course,
you are a special snowflake etc; but at the end, the majority of programmers do repeatable, simple work, e.g. writing line-of-business Java applications, maintenance work like adapting Cobol code to work with new tax rules, writing generic GUI's that work just well enough to justify the expense given that the app has only a few 10's or 100's of users, etc.
For all of these things, when managed properly (i.e., rotate programmers enough, take care of documentation, have strict coding standards etc), you can get a new programmer who has had a 2 year degree in programming and 6 months of training on the specific technology up to speed in such a project in a week. That's not quite as interchangeable as a guy turning screws in a car factory, but it's not that far off, either.
I realize that everybody likes to thing that they are so special and that without them the world (and certainly their company) would grind to a halt), but for the vast majority of programmers, it's simply not true. And for those where it is true, there is a large part where it's only true because management allowed one person to become so entrenched in one place that they've made themselves indispensable. And not because of the nature of the work.