1. They all came about when the Internet vastly increased the size of markets. The article makes good points about how many industries only have a few big players - that's kind of a natural consequence where when there is a low barrier to entry, the best only have to be a little bit better than everyone else to win out. Before the tech giants there were very few companies like Google and Facebook that were dominant in so many countries around the globe.
2. The discussion of changes to antitrust law was a good one, but I think what also happened was that tech giants were able to take advantage of entirely new business models to "confuse" how antitrust rules were targeted. E.g. even if you go by the "must improve consumer welfare" rule of antitrust law, many of the tech giants won because regulators were (at least originally) confused about who "the consumers" were. E.g. for Google and Facebook, as is often widely pointed out, "the consumers", i.e the people that are buying a product, are the advertisers, not the end users. Antitrust enforcement made the major mistake of only considering welfare of the end user.
3. Related to the first two, it is nearly always the case that when there is a big, transformative new technology that comes out, a few players are able to win, partially by moving faster than they can be effectively regulated by government, and which point the government is wary of doing too much harm to them because so many people and businesses depend on them (they become "too big to fail"). Just look at the early years of the auto business - there were a plethora of auto companies in the early 20th century, and after the dust settled we were basically left with 3 in the US: GM, Ford and Chrysler. And even then GM and Chrysler both eventually only survived through government bailouts (at least 2 government bailouts in Chrysler's case).