That's just late stage capitalism.

The harder and more complicated something is the bigger barrier to entry there is to competing against it.

When video games were simple and fit on a single floppy disk or tape - a single person could develop an entire commercially released game. John Romero could make Dangerous Dave in a week or two, by himself.

Now that games are like Grand Theft Auto V they require hundreds of millions of dollars to create with teams of hundreds of people over nearly a decade. The voice acting in motion capture alone cost many many times more than a game would cost to make in the '80s.

The same goes with web browsers. Chromium is open source and free, it works well, so why spend tens or hundreds of millions of dollars to make your own new thing?

What benefit did Microsoft get from spending all that money on EdgeHTML versus just using Chromium? None. That's why they switched to Chromium.

Oh... so to answer your question no one is "allowing" a few tech companies to denominate, just the complexity and cost of creating new products leads to these natural monopolies sort of forming. You're free to spend the tens of millions of dollars to make your own browser if you want to and break up this domination. I doubt you'll do it you'll probably just use Chromium.

Have you tried developing your own web browser?

The Web has become so complex, you need a huge team of talented developers to keep up with it, and for that you need a lot of money.

Robert Bork:

He also became an influential antitrust scholar, arguing that consumers often benefited from corporate mergers and that antitrust law should focus on consumer welfare rather than on ensuring competition.

Closest these days is an extension for Firefox that enable progressive web apps.