The person making $120,000 is heavily benefiting from a public infrastructure that makes $120k/yr jobs possible. A ditch digger needs almost no infrastructure besides a shovel to do his job. An engineer needs schools and universities to educate his colleagues, roads and railways to deliver parts required for the product he's working on, and police services that deter theft of those products once they're built. Not to mention a consumer base with enough money to afford the product in question.