It turns out that The Legend of Zelda: Tears of the Kingdom has been done for about a year, with the rest of that time being spent on just polish alone.
Back in March of 2022, Nintendo made the decision to delay Zelda: Tears of the Kingdom out of its 2022 release window into spring 2023, where it's obviously now out in the wild (pun not intended). It had always been made clear that 2022 was the target release year for the game, but wasn't a guarantee, so the delay wasn't shocking. Now, according to The Washington Post's Gene Park, the reason for that delay wasn't because the game wasn't technically ready; it was to polish it up.
Park recently spoke with Zelda series producer Eiji Aonuma as part of an interview with The Washington Post, but shared an anecdote that didn't make it into the final piece. "Eiji Aonuma said when he announced in March 2022 a delay for Zelda Tears of the Kingdom, the game was pretty much complete," Park shared in a tweet. "The last year was spent on polish, making sure the wild physics of the game just work."
Very often game delays occur simply because they just aren't ready to go out, with release dates more being set to please executives than anyone else. They're also becoming more and more common as games get bigger and bigger, but clearly it was worth it for Nintendo to delay the game by such a long period of time; it is the highest rated game of all time on OpenCritic after all.
Tears of the Kingdom has also sold incredibly well, amassing more than 10 million copies in just three days, beating out last year's big hit Elden Ring, and putting it on par with Pokemon Scarlet and Violet, a series which always sells exceptionally well.