The myth that you can’t build interactive web apps except as single page app

I am no frontend-guy, so I don't understand why in the age of node.js web-servers this ditchonomy exists between server-side and client side (SPA). Can't you initialise/pre-render most of your your stuff on the server, serialise it and push it through the client, which then acts as an SPA already initialised and then updates itself on its own. After all, both are JS? Why is the decision not more flexible where to run code, depending on latency, compute intensity etc. Maybe someone can enlighten me, as this is often left out, probably because it is obvious to someone working with these technologies.