The biggest problem with this, is that most of the big franchises didn't start off as big franchises and only became such things when hundreds of other IPs around them floundered or outright failed. Instead of looking to make a successful game/book/movie/whatever that they can hopefully turn into a franchise... they want it to be a big franchise from day 1 which usually doesn't work, comes at the detriment of the entertainment product itself, and sometimes oversaturates a market with something people don't give a shit about.Besides, inventing new intellectual properties is hard work, requires genuine talent and creativity, and there's no guarantee even a genuinely good/compelling idea will translate well to the target medium and actually be a success (the public is a fickle beast). Much easier to shit a "new" idea out every ten years and shovel it into as many generic games as possible in the hopes of scoring a rare winner.
For every Final Fantasy, Pokemon, Gears of War, Resident Evil, etc. there have been hundreds of other games over the decades that never did well enough to get sequels, or were simply shit, and they're the majority of games that are forgotten about. Hell, even some franchises that did get sequels still got forgotten about for the reasons you mentioned regarding people getting bored, or just moving on.
It's because they're treating videogame production like gambling at high stakes tables in Vegas. It's the same with hollywood and the out of control budgets. They're willing to drop 200 million dollars or more at a time in the hopes it'll make them 1-2 billion dollars or more, and give zero fucks about a 30 million dollar game that can make 100 million in sales just due to being a good game.I still will never understand the need for Sony or Microsoft to want to franchise everything. Not everything is going to be Gears of War, Halo, Tomb Raider... Just let it be a stand alone experience and learn from it to create another game.