Rendering server-side HTML is fine, as long as your application is stateless. Then it's really easy. However, when your application becomes more interactive and thus state-driven, is it really easier to it server-side? Remember, the primary function of modern frameworks is to have a declarative way of creating your by writing it as a function of state. Things get messy really quickly once you start to build something more complex.

Also, is it really easier to everything on the server? I don't think it is so by definition. You could argue that a stateless REST API with client-side state management is less complex than doing everything server-side. It's a much more scalable solution and creating interactive user interfaces is very easy.

Ofcourse it could be that interactive user interfaces is not something you are aiming for. That's fine. But do understand that most applications are actually usually very interactive.