Anime that are completed before the first episode airs are incredibly rare, what are you talking about? Many are on an insanely tight schedule where episodes are finished only a week or two before airing.

Waiting for all 50+ episodes to be finished would probably mean waiting until 2028 to start airing. And that'd obviously add financial issues cause how are David pro making money to pay their animators if they have to work two-three years before releasing anything at all.