Underneath his glossy wood desk, Jimmy Fallon is not wearing sweatpants. “I don’t think anyone wants to turn on their TV and see a late-night host in athleisure,” Mr. Fallon’s stylist Brian Coats explained.

Mr. Coats has dressed the “Saturday Night Live” alum since “The Tonight Show with Jimmy Fallon” on NBC debuted in 2014, and that means just one basic formula: suits. At the start of each season Mr. Coats organizes “a million different shirt-and-tie combos,” and racks up tailoring options from the likes of Gucci, Dior, Tom Ford and J. Crew. “There’s a respect and an honor to being in that chair,” said Mr. Coats, “and that’s how you show it: You wear an amazingly tailored suit.”

Late-night hosts wear suits: Such has been the custom since Steve Allen first sidled up to “The Tonight Show” desk on September 27, 1954. Back then, of course, most men routinely suited up. But even as the concept of TV itself continues to evolve in our Netflix-Hulu-tablet era, the custom among hosts persists. Even as politicians shed their ties and CEOs don fleece vests, these anchors remain almost absurdly allegiant to their inoffensive suit-and-tie uniform. From Andy Cohen on Bravo to Stephen Colbert on CBS to Trevor Noah on Comedy Central, they toe the line.

Tradition certainly keeps most hosts from breaking the mold. “There is reverence for the late-night format, part of which is the suit,” wrote Eric Justian, the costume designer and stylist of “Late Night with Seth Meyers” over email. Audiences expect late-night hosts—no matter how silly or satirical their comic barbs—to gird themselves in a suit, and an austere one at that. The default fabric choice is solid navy or black, though a particularly daring host might don a pinstripe, one so narrow you’d only detect it if you were glued to a cineplex-sized flat screen.

You can also trace the persistence of the late-night suit to its success as a comic foil. From Mr. Allen’s day on, the curtain would raise and out would step this man in a perfectly presentable suit. And then Mr. Allen or Mr. Carson or Mr. Letterman would open his mouth and a stream of oddball quips pour out. The contrast between the corporate-stiff attire and the droll humor added an extra wrinkle to the jokes. Here was a man dressed like your humorless boss, acting like a national jester. The jokes didn’t match the clothes, and the clothes didn’t match the jokes. The incongruity made for absurdist comedy gold.