Most major car companies and executives are sitting out the show, bypassing an event that was once one of the largest yearly gatherings in the auto business.

Some industry executives point to waning auto show attendance more generally and the need to spend their marketing dollars elsewhere, including on virtual and stand-alone events that they say resonate better with consumers. Other manufacturers say they don’t see a need to drum up interest because dealerships are already short on vehicles to sell, because of ongoing supply-chain disruptions.

This year, 15 automotive brands plan to have displays, down from 24 in 2019—the last time the auto show took place in Detroit—and 35 a decade ago. The show is also light on splashy new-car reveals, with only a handful of brands having scheduled news conferences.

“The days of the Detroit auto show having 60 new-vehicle reveals are over,” said Thad Szott, president of the Detroit Auto Dealers Association, which runs the Detroit auto show. “The way the companies reveal their products is completely different today.”

Major auto shows in the U.S. and Europe had been shrinking before the Covid-19 pandemic, as car companies sought to generate more buzz about new models through YouTube videos and social media. New-car searches today often begin and end online without a single visit to a dealership or even a test drive, dealers say.

Tesla Inc.’s success with revealing new models in boisterous, company-hosted events led by Chief Executive Elon Musk have also nudged more car companies to emulate this approach.

The shift accelerated during the pandemic, with many executives finding they could still reach a wide audience by unveiling their vehicles in virtual gatherings. Other international car shows, such as those in Geneva and Tokyo, have been canceled in recent years or have drastically altered their format.

Mr. Szott said show organizers are trying to adapt to these trends.

The emphasis at this year’s Detroit show will change from onstage reveals and exhibits packed with shiny sheet metal to test drives and other experiences, he said. Those include demonstrations of air taxis, or eVTOLs, which is short for electric vertical takeoff and landing. A giant, 60-foot tall rubber duck will also be positioned near the exhibit hall, a nod to Jeep owners who have a custom of leaving small rubber ducks on one another’s vehicle.

The show is still likely to have some big moments. President Biden is expected to give an address on the show floor Wednesday, the media day for the event, and promote his support for electric vehicles.

Global auto maker Stellantis NV has two vehicle reveals planned: one for Chrysler on Tuesday evening and one for Jeep the following day.

Ford Motor Co. plans to show off its next-generation Mustang sports car Wednesday evening at an outdoor event near the convention center where the show is held.

The annual Detroit gathering, officially named the North American International Auto Show in 1989, will be held in September for the first time, a departure from the frigid January event that had endured for decades.

The Detroit auto show had been canceled for the past two years because of Covid-19 precautions, and organizers moved the event to this month, following years of discussion about how to make coming to Detroit more appealing to visitors. In 2019, about 774,000 people attended the show in January, down only slightly from about 800,000 five years earlier, according to the show’s organizers.

Some auto makers say they remain committed to auto shows and may have a presence at other venues, such as shows in Los Angeles and New York.

“We know auto shows have been a longstanding industry tradition, and we remain committed to working with our dealers to engage customers in new and innovative ways,” a Honda spokeswoman said. Honda said this year that the shortage of cars and trucks on dealership lots was one factor in its decision not to participate in the Detroit show.

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GM plans to have full displays for its four U.S. brands at the Detroit show—Chevrolet, GMC, Buick and Cadillac—as a way to support its hometown show and local customers, a spokesman said. But it chose to reveal an important new model, an electric version of the Chevy Equinox SUV, last week with an appearance by General Motors Co. Chief Executive Mary Barra on the CBS Mornings show.

Auto makers have gravitated to their own events partly because they don’t have to share the spotlight with other companies, said Jason Vines, a former Chrysler Corp. public-relations executive who planned many auto show reveals.

Still, solo reveals can’t replicate the buzz and media attention created when one of the world’s largest industries descends in one place, he added. Mr. Vines helped plan a stunt in 2007 that included a herd of more than 100 cattle rumbling down the street outside the exhibit hall to mark the introduction of a new Ram pickup truck.

“Auto shows were important as a celebration of the industry,” he said.