They aren't "getting away" with anything. They keep their prices stable because consumers continue to buy their games at those prices.Nintendo's first party game quality output hasn't decreased. With that being said, I continue to remain perplexed how they can get away with keeping MSRP on all their first party titles throughout the lifecycle of their consoles, long after they are around as well. It's astounding. As someone wanting to catch up to the Switch library when I buy the new Switch model later this year, it's going to be expensive.
No publisher reduces a game's price because of some sort of natural rule or moral imperative to do so. They lower prices because that's the way to keep their games selling. If the market supported other publishers maintaining their initial MSRP, those publishers would do so.