How would the EU block them? This is especially important if the Russian or Chinese entity has no locus or presence in Europe.
(for instance, would you be able to get around it using a different DNS, or by using a VPN?)
A distant third problem are US regulators, which are slowly moving forward with targeting the tech giants (which now has both sides of the political machine looking at them, for different reasons). The giants no doubt feel like they can massage that situation better, as it's their home turf. They're invasive aliens as far as the EU is concerned.
I don't think it's surprising the global economy is littered with long-standing monopoly with little competition. It's not merely economies of scale, vertical integration, and other benefits of size, but because the business environment perpetually gets harder and harder for entrepreneurs to operate in.
The growth in the size of modern state-capitalist nation-states naturally coincides with the growth in monopolistic mega corporations - at the expense of things that have made capitalism so powerful and effective (competition checking abuse of customers, rapid innovation, etc).
The rat pelt bounty in French Vietnam led to rat breeding. The US luxury tax led to the wealthy saving more money and mass layoffs in the luxury industry. Bank regulations in the wake of the financial crisis gave big banks a competitive advantage over small banks.
These aren't apples to apples examples, but they show how regulatory action often has unintended consequences due to the complexity of the real world.
> Google can sit back and stop worrying
Name one company from EU who have a slight potential to be a minor threat for (YouTube / Search)
Even if you pay Google €0.01 per click, you still can get an impression under certain conditions.
If the European ad-buyers are funnelled into a subset of ad auctions, the average prices in those auctions should be higher.