I'm not sure they actually are extremely high. It's just that most other salaries have fallen below what we'd normally consider middle class.

Stated another way, the things that software engineers can do with their wealth generally seem like normal middle class things. They can own a home but they can't afford a yacht. They can take nice vacations but they aren't part of the jet set. They can start businesses but generally not in capital intensive areas like resource extraction or heavy industry.

I'd say that software engineers, at least the higher paid ones, are probably on the higher side of middle class; but they are still solidly middle class.