Why does it matter if they can? Do we allow unregulated oil drilling startups in the name of competition? There's no particular reason to assume that the platforms that govern our social interactions don't fall into a category of institutions who need to prove that they can keep a grip on their activity.
The quality and civility of public discourse is of principal importance to the health of liberal societies. Social media platforms are not irrelevant places of entertainment, it is becoming increasingly clear that they are the primary space where public consensus is formed. As such, they should be held accountable accordingly and not be treated like some random market provider of cat videos.