Those wireless keyboards you mention all require connecting a USB transceiver into an available port. The TV only has one port, and it's designed for use either for media playback or being able to pause live TV. While the Roku OS is based on Linux, the kernel is customized for the OS and likely has removed the ability to connect a remote keyboard. It's done to simplify the OS core functionality, reduce memory requirements, and make it more secure.
Those wireless keyboards cannot connect to the existing 2.4 GHz WiFi connection. They're the same frequency band, but use different protocols so one can't be used with the other.