Monopoly is rarely the correct word. MLB is a monopoly. Commercial brewing in America is an oligopoly, the economic effects of which are not substantially dissimilar to those of a monopoly.
AB Inbev currently holds something like 39% of the domestic market. MillerCoors holds another 26%. Heineken and Modelo bring up the rear, with about 13% between them. That's 78% of the domestic beer market shared between four producers. The remaining 22% is shared amongst the thousands of smaller commercial breweries around the nation.
Just because you and I can make our own beer and maybe even sell it about locally doesn't mean that we can actually realize commercial success, particularly when distribution is so heavily skewed in favor of large breweries.
Wes þú hál, Beowa!
Primary: Little Lion IPA v2
Bottled: Little Lion IPA (v2), Proud Kitty Pale Ale, and one last bottle of the weird brown ale that wanted to be an Oud Bruin when it grew up.
On deck: Cascadia, a CDA, or a stout.