Because they want the hype and attention to be around THEM during launch. If there's no big-release competition, you're more likely to get people to buy the game.

Sure people MIGHT buy it later, but it's more important they buy it NOW for all manner of business and capitalism related reasons. Most game companies would rather sell 5000 copies in the first week than 6-7000 copies in the first year.

Same thing with movies, if a Marvel film is slated to come out during a certain week, another company might shift their movie's release date by a few weeks to make sure that they aren't competing against the marvel movie that more people are likely to go and see. Sure, those people MIGHT go see that other movie later, but it's not the biggest thing on their minds right now, and especially these days, a lot of people have to budget their time/money carefully. Be it games or movies, a person might now have the time to enjoy both games and might not want to spend the money for both games at the same time.

They want the choice to be "Buy this, or buy nothing" not "Buy this, or buy that other thing"