Quote Originally Posted by Eorzean_username View Post
I think (and anyone who's better-informed, please correct any details I get wrong) that it's an intentional feature of the game that was added [citation needed] time ago as compensation for the game's dubious handling of client-server relationships.
I think it's just how casting has always worked. Even in FFXI, since 2002. But it wasn't really something people used like a mechanic there since that game is so much slower paced and doesn't have manual dodging. But for most spells you could usually move once the cast bar was past ~70% and not interrupt yourself.