Creating a server devices can access on the same network is trivial the only issue you would run into is maybe what port to use and there are discovery processes for this.

For that to work, you need a device which is online 24/7, always stays on the network, and does not have energy efficiency requirements, which justifiably limit background app activity.

The only (consumer) devices that fulfill these requirements are routers and some smart gadgets, like fridges or smart speakers. None of them are easily programmable to a sufficient degree.

This still doesn't solve the problem of sharing among users on different networks and accessing your data when you're not home.

All these problems are solvable, but they'd require massive coordination among device vendors, and coordination is one of the hardest problems out there.