Modern Browsers are capable of doing the heavy weight lifting - Rendering the UI! They aren't the dumb terminals anymore that simply display whatever the server sends!
This point also makes Server-side rendering is the technology of the past viable, somewhat! :P
Meteor makes use of this fact and sends "Data over the Wire", not HTML. And the meteor front-end on the browser consumes the data and takes care of the rendering the UI.
I think that's the reason why Meteor doesn't support server side rendering, quite yet! :)
btw, there are some attempts to bring SSR to Meteor: meteor-ssr
But hey, again, why to let the server do all the heavy lifting when the browser can jump in do it? Let the server have some fun! :P