Because a lot of society buys into said consumerism and expects bonuses and stability heading into lots of potentially irresponsible gift-buying decisions for their friends and family?
And I'd expect the calculus is "we're planning to offer severance and paying out bonuses, so they have stability in their consumer choices, and we can start the clock earlier on their severance by doing it as soon as possible (including during a holiday when those employees probably weren't going to be very productive anyway)"
Because the made up consumerism focused holiday is celebrated by most Americans and generally involves buying gifts for loved ones and attempting to relax with family.
Since most people use the money and PTO from their jobs to enable these goals, losing one's job during this time of year adds an extra layer of stress to those who might have hoped to celebrate the culturally dominant evil made up consumerist capitalist holiday with their loved ones.