Because the medium is different. Instagram is different images, twitter is text, Fb is more local social circle. But if you have 5 twitter clones, no one wants to use 5
The thing is, Fediverse could be all of those things. There's an Instagram clone, there's a YouTube clone, there's Mastodon that's similar to Twitter, and Friendica that's more like Facebook. And you can follow them all from a single account if you want.
They could but hard to compete on content
YouTube is the big one there. But you can still share YouTube content on the Fediverse.
Even if the medium is different, the original statement was "no one wants to check 20 apps for 20 social media...". Which, as I've shown, is wrong, because already people do check 20 different apps for 20 social media accounts.
If anything, activitypub fixes this. The whole idea of the protocol is interoperability: that you need only one app to check all of them. Whether they are 5 twitter clones talking AP, or a Linkedin, youtube, meetup, and twitter-alike talking AP: all are interoperable, and you can interact with all of them from just one account and app.
Then why do we have Linkedin, Twitter, Reddit, Youtube, Orkut, Whatsapp, Instagram, SnapChat, QQ, Facebook, Hackernews, Discord, and Stackoverflow?
What you say people don't want, is exactly what people already do.