It would tie in an additional feature with potential future incompatibilities, and it moves insecurities in templates to the client runtime where they’re more dangerous than compile/interpret time.

Eg some template feature creates a security issue and has to be disabled, which invalidates all the old HTML that used it.

There’s also just not a super great reason to do it. It’s not all that different from pre-templating like Hugo or doing it server-side like Django. You get to offload some compute, but HTML templating isn’t usually the bottleneck.

It's a markup language for documents. If you enable templating devs are then going to ask for more. Eventually it will end up turing complete. Such is the development arc of a lot of languages.