Why use A2UI instead of Streaming MDX?

It seems models are really good at outputing MDX, it's Markdown + JSX after all. You just tell model which Components exist (e.g. Column, Row, Card, Form), show it some examples and it does really good job, even smaller/older models.

OpenAI's new GPT-6 Intelligent UI uses streaming MDX with custom inline language for React style hooks.