How did we end up with Frontier Lab oligopoly rather than a monopoly?

Frontier labs - OpenAI / Anthropic / Google Deep Mind

Wireless - ATT / Verizon

however,

Search - Google

Videos - Youtube

Social networking - Facebook

Microblogging - Twitter

I know there are competitors for all the dominant players in my list, but the market share distributions are very different imo.

Is it complete randomness that markets converge on monopolies vs. oligopolies? Or is there something principled or systemic behind that?