The cost and time investment involved with producing anything on a company level has to be weighed up.
A hobbyist graphic designer doesn't have to think about this sort of thing.
The reason is less about the fact that they can't but more about the fact that they are not willing to invest the resources into this.
What's more strange to me is why they didn't produce the textures in high res to begin with, and scale them down. But I am by no means versed in game design.