There has been a lot of debate in IT about this, but, for example, when it appeared Microsoft had achieved monopoly status in a broad range of IT areas, the whole landscape shifted rapidly, to the Internet, to Google, Apple made a big comeback with personal systems, iPods, iPads, iPhones, and Facebook capitalized on a new paradigm of what Internet customers wanted. Microsoft very quickly looked like it's better days were behind it, no longer a serious monopoly threat.
All of this happened too quickly for regulators and lawyers to address the previous monopoly situation. Though Silicon Valley has some of the markings of the Robber Barons, things change so fast that by the time actions are taken to address monopoly power, the entire landscape is completely new.
The Europeans are much more aggressive in trying to hold dominant Tech companies to account (Microsoft, Apple, Google), but there's very little evidence it's having the desired impact.
At least that what it looks like to me.