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.