I would think a primary reason for that is performance. A key that large would require a lot of entropy for initial generation, a large(r) amount of memory, as

I would think a primary reason for that is performance. A key that large would require a lot of entropy for initial generation, a large(r) amount of memory, as well as making encryption much more computationally expensive. I'd also be worried that with such large keys there might be greater potential for side-channel attacks.