That assumes the server has a lot of additional CPU power to serve the content as HTML (and thus do the templating server side), whereas with XSLT I can serve XML and the XSLT and the client side can render the page according to the XSLT.
The XSLT can also be served once, and then cached for a very long time period, and the XML can be very small.
With server-side rendering you control the amount of compute you are providing, with client-side rendering you cannot control anything and if the app would be dog slow on some devices you can't do anything.
The XSLT can also be served once, and then cached for a very long time period, and the XML can be very small.