You're absolutely right in the sense that the Django and other communities should not stagnate and should also keep an eye on what the other frameworks are up to. Maybe the default template engine should be replaced in Django?
Template engines are already configurable and pluggable for quite a while.
The "problem" is that most of the third-party apps that want to provide templates end up using the default, so either you have to reimplement those or stick with the default engine.
You can use both the Django Template Engine and other template engines side-by side in the same project. You can then write your custom templates in whatever you want, and have your third-party apps use whatever template engine they want to.
The "problem" is that most of the third-party apps that want to provide templates end up using the default, so either you have to reimplement those or stick with the default engine.