Because we have more craft breweries than probably the rest of the world, combined:
Today we have 1,759 brewing facilities—more than the pre-Prohibition high of 1,751. What happened? People got sick of corporate swill and started brewing their own. Some of them got really good at it and started small breweries. The circle of people who enjoy a good beer widened, drawing more good brewers in. And so on.
... At any rate, the explosion in popularity for real beer has been noted been by the giants, which is precisely why they can longer just peddle name brands like Bud and Miller but also have to roll out phony craft beers like Anheuser-Busch's inglorious "Shock Top" line. And as people consume less flagship swill like Bud and Miller, the companies have responded by desperately buying up smaller regional brands in hopes of keeping as many consumers as possible. Hence the rapid consolidation.
I think it may be time to review and amend the brewery crawl a friend and I did in 2009.