Because its almost impossible to compete? These companies have so many resources, its impossible for any local competitor to compete. Amazon can just crash the prices till the competition dies, Google can just not allow YouTube on it, Facebook will exist because of the network it has.
What the EU is doing is what is needed to happen long ago. These companies are not currently successful because they offer the best experience or the best innovativtion, they are successful because they crush anyone else.
For example WhatsApp has many many better alternatives, which have better features and better privacy, but it still the #1 because of the monopoly it has on communication.
What the EU did here is smart, they didn't outright ban WhatsApp, or funded a direct competitor. They forced them to play fair, to stop the monopolistic behavior and force them to compete on features, rather than succeeding only because my familly is on WhatsApp.
The same thing applies to Apple, which forces to everyone to use its crappy, intentionally handicapped browser engine.
And also forcing everyone to use its payment services while taking percentage of the profits and not even allowing you to increase the prices to cover their percentage!, this is absolutely outreagous and finally something is being done about it.