Because it's relatively unskilled work that requires mainly a strong back and minimal training. Look around the job site and see who's making money. It's usually the managers and skilled tradesmen. The more you know, the more your paycheck grows.
An unusual serious comment from me, but seriously, join a company, I'll use a building company here, and start off by moving bricks around, then you move onto laying bricks and adding insulation, then you start to focus on a certain area, eg, carpenter, brick layer, scaffolder, fitters. And if all else fails, be a shrink