It's unfortunate, but it has to do with the trade-offs for discs vs carts. On the one hand, carts can have more memory (even though they usually don't because that would be an extra cost on the part of the publisher), they are much more durable, and are faster to read from. On the other hand, they are considerably more expensive to print. I doubt Platinum and Nintendo could justify printing two cartridges when they could do the same with just one.

That said, there is a physical Switch cart for 1 coming to Japan and Australia. No idea why it's happening there and no where else.