They would need to extend the slots from the existing ram, which would slow it down even when empty, or they would need to use up a lot of silicon adding more memory channels. It really does make sense to solder on RAM for best performance. Though alternative sockets like CAMM might help in the future.

But you can't do a trade-in, the RAM they sell is extremely overpriced, and as mentioned elsewhere there's no excuse for the SSD.