Seriously, it's a component of costume design. It has been since before there were comic book superheroes. Steve Ditko used it, Jack Kirby used it, Dave Cockrum used it, Neal Adams used it, Jim Lee used it. In most cases, it's a way to break up the colors of the lower body; in some cases, it's just to add seams, apparently. What it emphatically is not is uncommon, or even necessarily a sign of bad costume design. Check out lists of the best costume designs, and you'll see some consistent entries: Blue Beetle, Green Arrow, Wolverine--all dressed in external underwear. Can we all please stop acting like Superman's red briefs are some terrible and unique crime of fashion?
I mean, Moon Knight wears white all year long. Seriously.