OK, others have asked for a wireless keyboard, which would imply it was offered by Roku.
The problem with connecting a USB keyboard is 1) not every Roku has a USB port and 2) the Roku OS would need to have drivers to support a USB keyboard. the Roku OS is a highly modified version of Linux. While the base OS certainly has drivers for USB keyboards in the kernel, drivers take memory space, and many Roku devices simply don’t have enough memory for things like drivers that aren’t necessary for the device to function. So Roku removes all unnecessary things from the base kernel, including drivers they feel are unneeded. They need all the space they can get to allow the Roku OS to work on a wide variety of hardware. Adding support for a USB keyboard (which I actually agree with) likely just won’t fit within the footprint they need to maintain.