Not trust others is pretty obvious: leaders push for long hours because they don't trust people to be intrinsically motivated or to work in the most effective way for them. If you assume people are inherently unmotivated and lazy, well, trackable hours and artificial pressure seem like the obvious consequence.
But it's also a sign of not trusting yourself. Being judged on outcomes—never fully under your control—is scary. Being judged on anything fuzzy or arguable—taste, experience, skill, insight—is scary. If you're the sort of person who is content to "grind", the best way to win competitions is to turn them into grinding competitions. You can't be confident that you are more skilled, more intelligent or have better taste than others, but you can always just "grind" that extra hour. For a certain personality, time spent is by far the easiest metric to control.
If you grow up constantly being praised for how "hard" (read "long") you work, constantly out-competing people by doing more rather than better, the inherent value of "hard" work over everything becomes fundamentally ingrained in your personal story. And, unfortunately, our culture tends to put those people into positions of power, so this tendency gets reinforced and propagated.
Taking a step back, doing something good with less effort ought to be more impressive than doing it with more effort. That's what real potential looks like.
More importantly, even if working more hours purely increased your effectiveness and productivity—and we absolutely know that it doesn't—it would still be a weak form of leverage. Maybe you can work 2× the hours, but you can never work 10x. On the other hand, with taste and experience, you can absolutely come up with a 10× better design, or a 10× better understanding of what you're doing, and, unlike long hours, those 10× advantages compound.
If you trusted your own taste and creativity to give you the leverage you need, you wouldn't work ridiculous hours because you'd know it's counterproductive.
But when you don't, long hours are an easy, socially accepted fallback.