Even if the SoC was finalized years ago, there's more to the system than that. For instance, microSD Express cards. We only got an announcement last year about their availability for consumers, and the article mentioning that had said that it was because of a certain "customer" of theirs that it became possible. This customer was likely Nintendo.
This will probably sound like defending Game Key Cards, but I just want to give my opinion on why they exist.
Switch 2 utilizes fast storage in order to make use of its hardware decompression block for near-instantaneous loading of compressed assets in a similar manner as the PS5 and Series consoles. It uses fast internal storage, and uses microSD Express. So carts meant to load games straight from them need to be fast as well. Problem is, that can't be done with 2D NAND that Switch 1 carts use because their design doesn't allow it. 3D NAND does because the memory cells are stacked vertically. Instead of reading each cell individually, it can read a group of memory cells stacked like a totem pole all at once. So 3D NAND is faster, but what it also benefits from is being cheaper at equal capacities. What causes a heavy cost of 2D NAND isn't the memory cells, but the base material they sit on. By taking that 2D plane of memory cells, and arranging them into a cube, it reduces the amount of the expensive material used for the base.
This is all good except for one factor. Manufacturing time. 3D NAND may be faster in read/write speeds, and cheaper to make, but the method of stacking those memory cells is very complex, requiring quite a bit more time per unit. So the situation Nintendo and Macronix (the makers of the memory) are dealing with is the current inability to produce a lot of memory chips. If they were to try and make them in multiple capacities, they'd run a greater risk of shortages. With games from major studios pushing for increased game sizes, the decision they made was to stick to one capacity that technically can handle the biggest games, the 64GB capacity. For everything else, they opted for those Game Key Cards. It's not the best method, but was one that was probably needed for those that don't want to be strictly digital. They can be traded, sold, etc. There's always the question about them becoming useless if Nintendo shuts down the servers, but so far, the only thing Nintendo has done is shut down the means of purchasing games on older platforms. Folks can still download their DSi and Wii titles to this day, around 2 decades later. I'd imagine that physical media would probably break down before they prevent people from redownloading their purchases.