They're not always feature complete. Plenty of games are crunching to finish things last minute and sometimes do last minute cuts because they can't finish things.
But I agree that they'll usually release at the announced date regardless of the game's final state. To be fair to them, advertising is insanely expensive and ad campaigns are often planned with a specific release date in mind. Some software is never gonna be 100%, too, and does need some pressure to just accept some issues or cut some things.
But usually it's just a plain old disconnect between devs and higher ups/project managers. Estimating software dev is insanely difficult and higher ups in charge of the final calls are often either not devs, too insulated from the devs, or sometimes just egoistic to listen to anyone but themselves.
Personally I started with kbin and think the dev of it is great. But it's simply not as far along IMO. At least when I was using it, it was critically missing the ability to collapse comments. That single feature is huuuuge for me and probably the most prominent thing that got me to switch to Lemmy.
It also doesn't have an API yet, which means that mobile apps aren't likely to target it. Though I've personally been using a browser cause I haven't found any apps to be good enough yet.
Also, the notifications of kbin felt very buggy to me. I missed a lot of notifications and even when they worked, they don't show the notification or even what the thread title is, so you have to click each one individually. IIRC, clicking the notification also didn't work if your comment wasn't on the first page of comments.