Because Rails, Symphony, Django, Phoenix or whatever does the job in the preferred language of the developer/team and often has more batteries or better tooling included.

I bet those are not a Front end's preferred language and you end up need JavaScript anyway if you need some extra client form handling. Now you use two language to do one thing and made it more difficult to be maintained.

That problem was solved approx 20 years ago.

The alternatives are not react and templates vs writing html by hand.

The browsers had support for XSL templates

You write your website in XML and the browser renders it using the template