I think it really just comes down to scale. Relative to other professions there aren't that many software engineers, but the work produced by each one has the potential to reach an extremely wide user base. Someone working at Google could write code that gets deployed on a billion devices. This is pretty clear when comparing between different software engineering roles as well. Companies that serve a global market pay significantly better than local companies.

On top of that, there's no supplies or logistics required for software engineering. It just takes one person and a computer, so expenses are minimal compared to other engineering disciplines.