First, because the goal of corporations is to maximize profits, not to maximize people's well being. The point of socialist government (and more generally of "correct" economic decisions) is supposed to be to maximize people's well being. For example, Google is a large corporation because of economies of scale in leveraging search and other apps to capture ad revenue. It makes no profit from serving its users' needs, except indirectly. Similar remarks apply to Facebook and other tech giants.
Second, because centralization in industry is actually dependent, to a large extent, on centralization in government. Large corporations have all kinds of advantages that have nothing to do with serving customers well, and a lot to do with being able to successfully lobby lawmakers and regulators to their advantage. The tech industry is young enough that this process hasn't gone as far as it has in, for example, the US defense industry, where all of the large corporations exist only because they have optimized themselves for getting government contracts.
In other words, large corporations don't have any magical way of getting around the information problem I described, any more than large governments do. They just latch on to a source of profits that doesn't require them to actually aggregate all that information at all.