I think it's mostly unique to knowledge workers, which is a broader category than just software engineers, but does not include most jobs. I think the effect is more pronounced in software engineering because of high demand, as well as the margins for software, like you mention. (Anecdotally, I have seen the same effect with friends in other industries, like electrical engineering and chemistry, just to a lesser degree.) With that said, I think it's significantly easier to put a monetary value on, say, an HR person or office administrator than an engineer or scientist.

There is also the issue that the value of two different knowledge workers, even at the same company, may be wildly different depending on their specific skills and the project they are working on. For instance, someone working on product features at a software company is much more valuable than someone maintaining an internal reporting tool, even at the same company and even if their skills and the type of work are nearly identical.

The point is, there are a ton of variables at play here, most of which can't really be measured, so everyone plays a guessing game to determine what engineers are "worth". The asymmetry of information also doesn't help, because it's hard to know how much you could make if you don't know what anyone else is offering—many people take salaries well below their potential and don't even realize it.