Part of it is how cheap and easy cloud providers have made assembling all the services you would need to host and scale the app, the other part is that an AI coding agent can do literally all the development and sys-admin tasks that would be needed to get it done.
It would take an experienced full-stack developer about a week or two of effort to direct and manage the development of the entire web app, plus mobile, plus infrastructure, including everything needed to set it up and launch it online. Given a "social network" is a well known category of app? It might even take as little as a couple days.
There's almost no reason for signups given the name recognition already that I can see. Throw the app together, put a "donate" button on the sign up page to pay for the first few months of hosting until ad revenue kicks in and be done with it.