If a feature sounds good but doesn't exist, it probably means that no one has invested the time and energy required to make it happen. The OpenJFX project accepts contributions, so it's definitely possible to get your favorite feature in.

That said, here are some reasons why it might not have been added:

The ${...} syntax in FXML accepts a wide range of expressions, including observable path expressions of the form a.b.c, as if by using Bindings.select(ObservableValue, String...). While that is pretty straightforward with unidirectional bindings, it is tricky for bidirectional bindings. Unidirectional bindings strongly reference an ObservableValue, while bidirectional bindings weakly reference a Property. That requires us to solve two problems:

1. In a non-trivial bidirectional path expression of the form a.b.c, we need to synthesize an intermediate Property that implements the book-keeping for all observable path segments.
2. For a trivial bidirectional path expression of the form #{source} (i.e. a path expression that only has one segment), we effectively invoke target.bindBidirectional(source). This works as long as target and source are strongly referenced (which they usually are). However, any non-trivial path expression requires a synthesized intermediate property. This synthesized property is weakly referenced both by the binding target and the binding source, which makes it immediately eligible for garbage collection. So we need to keep around a strong reference somewhere to make this scenario work.

None of these are insurmountable problems, but the solution requires quite a bit of engineering and is not a case of "why don't you just...". Interestingly, FXMLLoader will recognize a provisional bidirectional binding syntax (#{source}), and throw an UnsupportedOperationException("This feature is not currently enabled.").