Most game development studios set release dates based on an estimated time they can complete the game. They also set release dates for publisher/retailers to have an ETA as well.
That doesn’t necessarily mean they will complete the game on that exact date but that’s what their goal is set at. Just like anything in life sometimes they run into snags in development and they need more time to complete the game, add more features, or to make a game better than a competitors.
For the most part, the only release dates that can be trusted are for console games, which know pretty much the exact development time they have and how much they want to add to the game before they release it.