They're not just more difficult to build, there are only two options in an infrastructure like those. You can build a system where everything depends on one ser

They're not just more difficult to build, there are only two options in an infrastructure like those. You can build a system where everything depends on one server (where if your internet isn't good enough everything breaks) or you use a Bitcoin-esque system to make sure that every computer on the network gets the messages. Which causes noise and inefficency. Needless to say, it's harder than you've stated.