It's an integral part of a consumerist society, it would seem. It's not like people have the innate urge to go out and buy superfluous [stuff] right after eating irresponsible amounts of Turkey and dinner rolls. Corporations like Walmart, Target, and pretty much every retailer have their own tradition of getting people to buy their [stuff] by lowering the price by what would seem like a ridiculous margin, even though they profit more from the sheer volume sold.
So, as the price goes down, demand increases and people (especially those with little disposable income) rush out to try and snag whatever they can get, whether it be for themselves or Christmas gifts that they may not otherwise be able to afford.
Could it be any other day between Thanksgiving and Christmas? Yeah, but they have their own "tradition," and if one retailer suddenly decides to change their sales to a different day or spread them out over a longer period, they'll lose that massive volume that makes it more profitable for them to begin with.
And, unfortunately, even if there are those of us who refuse to participate, we'll likely be a minority for a long time and, unfortunately, be dragged along by the will of the majority (that same majority that elected Trump as president).