Let's take NextDoor for an example. It's a good idea and it used to be a better site but it has gone the way of enshittification. If you tried to build from scratch a competitor which is more like the original version of the site, what kind of troubles would you run into? This is not specific to NextDoor it could be any app or service that got ruined.
A "platform" is a combination of users and software. You could possibly build the software but how many millions of dollars did NextDoor spend to get and maintain the initial set of users? How would you get that initial investment and then an ongoing revenue stream to grow the size of the user base?
Without millions of users, the NextDoor software isn't useful.
Without millions of users, the NextDoor software isn't useful.