I object to calling these organizations conglomerates instead of brewers.Let's take that last paragraph first, because it's instrumental to how we think about this issue. I would argue that large beer companies are categorically different from smaller breweries. These companies manage large portfolios of products that they manufacture in industrial plants. They produce a variety of beer that varies little by brand or region, which makes it very easy to create new brands, sunset others, all on a massive scale. Small breweries are associated with single brands and are not optimized to produce consistent beer in large quantities. (As with all things, things get fuzzy in the middle.) I say they are categorically different because the production methods and products of large industrial breweries, whether we're talking about Dutch Heineken, Mexican Modelo, or Chinese Tsingtao, are very similar to one another and very different from cask ale breweries in England, saison breweries in Belgium, or weizen producers in Bavaria. They're optimized to make interchangeable light lagers, and as they acquire and sell brands, that's exactly what they do.
A conglomerate is an entity who's various enterprises are not related. Example: General Electric. Owns TV stations, manufacturers jet engines, offers financial services, manufacture oil equipment.
AB InBev and SAB Miller, by contrast, are more or less exclusively focused on the design, manufacture, and sale of beer. They are highly specialized, with massive expertise in the beer industry. They are practically the precise opposite of a conglomerate.
You many not personally be fond of their methods, or their products, but they are brewers, and to call them otherwise is simply inaccurate.
This distinction is far from unique to beer. Most food and drink markets have an expensive, artisanal end and a mass-produced, industrial end. In a very real sense, they're not the same markets. It's true that Kraft and Rogue Creamery are both in the cheese business, but the people buying American cheese by the slice are never going to spend eight bucks a pound on Oregonzola--and vice versa.
I don't think anyone doubts that many (most?) big beer companies produce impressive products. They're extremely consistent and clean--and therefore very hard to brew. When I was traveling through Britain and Belgium, brewers there made a point of praising Anheuser-Busch to me for their level of accomplishment. But it's fundamentally a different product than what Frank Boon or John Keeling (Fuller's) make.
Now, as to conglomerate, Wikipedia backs up the definition from the commenter. I'm not sure if it's a universal definition, but I think it's linguistically useful. The commenter's right--mostly those companies are in the beer or at least drinks business (like Diageo). But I also think it's important to realize that in using industrial-scale breweries optimized to produce interchangeable light-lager-type beers, they're a different beast than small breweries. So what: agglomeration, maybe?