They can't do the cafeteria worker's job or the plumber's job or the security guard's job either. Maybe they could learn over time, but they couldn't slot into any of those positions and be any good at them on day one. Chances are they'd be tapping out on day two. Specialized labor is effective for a reason.

Or maybe you're just really fucking snooty and you completely devalue what it is custodians do with their time. In which case, consider that a company would run way longer without a CEO than without any custodians.