Because on paper the barrier to entry for social media is very low. It's literally just a website.
On paper, the barrier to entry is staggeringly high, which is why Facebook effectively has a monopoly on text-based social media and (via Instagram) photo-based social media, and you have to include radically different things like video sharing (short-form and long-form), private messaging, and microblogging to be able to claim that it has any competition at all.
But those things are really fundamentally different types of communication that appeal to fundamentally different audiences for fundamentally different reasons. And while they might be "competition" in that both take up your time, that's a bit like arguing that TV news competes with gym memberships. In any sane universe, they should be treated as entirely different markets. But Facebook has managed so far to convince judges and juries that they are all "social media" to avoid antitrust scrutiny, despite having killed the only viable competitor ever to exist (Google+).
The reality is that true competition in social networks — social networks fighting for the same eyes — is basically impossible unless you have government-mandated federation between social networks. What happens instead is that everyone of a particular age suddenly gets old enough to join social networks, and they join whatever is popular with young people at that point, because everyone they know is on that network. About once per generation, that social network starts being seen as "the social network for old people", and some new competition has a chance of taking the new folks. And they compete for a year or two until one becomes dominant, and then the market becomes static again.
So you get a brief moment of competition every decade or so. And that's it. The rest of the time, your choice of social network is dominated by network effects, where people choose a social network almost exclusively because everyone their age is on that network.
Can you imagine if someone said, "I'm going to create a competing telephone network that doesn't talk to the existing telephone network?" Everyone else would laugh in their faces. Yet that's exactly what social network competition is like.
So no, the barrier to entry is not and has never been low. That's why one of the wealthiest companies on the planet tried to compete head to head with Facebook and still couldn't pull it off. Anyone arguing otherwise is depending on a Frankensteinian hybrid market that treats competition as being between companies instead of between products. Facebook Reels competes with YouTube Shorts. Facebook Messenger competes with dozens of other companies. Facebook Groups competes with Discord. Facebook (as a friend-based text sharing social media platform) doesn't compete with anybody. Instagram (as a friend-based photo sharing social media platform) also doesn't compete with anybody meaningfully.
Put another way, the barrier to entry is low if you can come up with a totally different type of content to share that no other social network supports, and that takes long enough to support properly so that Facebook won't duplicate your feature and kill your momentum by week 6. Otherwise, network effects combine with monopoly market power to make the barrier to entry startlingly high. It is the "social" part that makes this true.