It's not done by the core team. It was a separate project by separate people that was brought under Semantic UI's umbrella. That's actually the major reason for the lagging behind: Semantic UI is designed for a circa 2008 jQuery website. It's a beautiful UI, but the CSS uses a horribly dated model which obligates child elements being aware of the parent elements.

End of the day, until it's rewritten with modularity in mind it will always be a drag to properly implement it in React.