This was a bit of an accident really.
Maybe one day big companies with monopolistic tendencies will be broken-up. But it will be a long and hard process.
Look at the Standard Oil monopoly of the late XIXe/early XXe century, it took decades to break it up, even with laws against it.
When the break-up will happen, I see two ways this could happen:
* either break by activity (separate AWS and Amazon, Gmail, Youtube and Search in Google, etc)
* break each service in a way to form a distributed system shared by several companies
For example, split Google Search with part of the search index and engine distributed across several companies, and each companies having their own offer in term of UI design, data privacy, additional functionalities, plugged on this shared index.
(*at the time, when the sector was much smaller)