It's a physical job. Men are on average stronger. Even though the distribution of strength might not be proportional, the general perception should lead to a manifest self-image simply by association. And it helps that people like me corroborate this, frequently.
Pure strength is not the only metric that is important on most physical jobs. Endurance is another metric and afaik women tend to rate fairly well on that. This is also a mentally tough job, working in confined spaces in the dark with low visibility requires a whole range of attributes other than mere physical strength.
