The Burn After Reading star had been due to attend the Grand Prix Ball Abu Dhabi in the United Arab Emirates on October 31, but failed to show up.
Photographs later revealed he had joined partner Angelina Jolie and their six children trick-or-treating for Halloween in Los Angeles.
A source told a magazine: "He's spending more time with the family and doing less of these appearances, even though it's a huge sum of cash."
A spokesperson for the event - which featured performances from Beyonce Knowles, Aerosmith and Timbaland, and raised money for clean water charity Fresh2o - admitted Brad had been expected to attend and was unsure why he had shunned the huge appearance fee.
James Magee, a representative for Global Event Management, said: "We were told he would be attending as the guest of the Tourism Development and Investment Company. But he was a no-show, and nobody seemed to know why."