Big techs are relevant primarily because there are no other companies that can 1. execute its business reasonably well 2. at the planetary scale. There are some other factors including anti-competitive business practices but that won't work without its own edge; MS was not able to stop Google and the same thing for Google to FB as well.

I've seen many HN readers claiming they can build a FB clone overnight, but that's not even the tip of iceberg. Serving your service to billions of people across hundreds of different languages, cultures geographical areas with different legal, financial, logistical infrastructures is at the fundamentally different level of complexity than building a toy project. This is not just for big techs, there are literally thousands of uninteresting but extremely profitable businesses simply for this very reason. And big techs owns platform which brings them to another level...