Mainly because they all discovered that fully dynamic client-side SPAs written in 100% JS are extremely slow, and wanted a way to speed them up whilst keeping m

Mainly because they all discovered that fully dynamic client-side SPAs written in 100% JS are extremely slow, and wanted a way to speed them up whilst keeping most of the developer experience of an SPA. That's it. It's not a clever way to arrive at the conclusion that the server is useful, but they got there in the end.