Because he believes in what they are selling so much! LOLz!
In all seriousness, it has come to our attention that Will Ferrell is in not one, not two but three new spots for Old Milwaukee beer. Now, it isn't odd for a celebrity to endorse crap they don't really like just to make a quick buck, but apparently, Will is such a huge fan of the beverage, he approached Pabst Brewing Co. to do the commercials - for FREE!
Of course, the company was delighted to have and essential, gave him creative control to do with the commercials as he saw fit. These are the fruits of his labor.
Check out the first of the commercials above, then click the jump to see the other two.