Interestingly - I'd like to add some cultural insight to this topic! Japan really loves events, it's one of their more interesting cultural differences. That's not to say Japan likes events more than the west, but they kind of see it differently.
So, there are a lot of festivals held throughout each year in Japan. A lot... They have a lot more "important" or "spiritual" holidays because of how shinto buddhism works. A more modern comparison, which might be a bit hit or miss depending on your interest in anime: You ever watch kids anime? Things like Megaman Battle Network, Beyblades, heck Pokemon... There are "events" with big idol presenters... things change on a dime, and the characters never know what to expect, but they're forced to deal with these "events" or sudden rule changes as they come. It's common theme. The thing is Japan loves the whole element of surprise. They look forward to these "announcements" which why we get the Squid Sisters announcements when we boot up the game! It's a big "OOOOH What's next?"
So in my opinion it has more to do with attempting to please a certain audience. Splatoon has a huge Japanese audience, and it was made by Japanese developers, and even features a lot Japanese elements, such as the Squid Sisters songs, The way the generic Splatfest T looks, and the Giant Tanuki and Fox on top of inkopolis. It might not make sense to a western audience, but to it makes sense in a cultural context. Even Monsters Hunter has "event quests" that give you a dinky little souvenir for completing them.
