I specifically mentioned those special cases. It's obviously nearly impossible to play RTS and MMORPGs with a controller effectively. And of course another reason we use keyboards is that some games are just text input/typing-heavy, it would be mighty inconvenient switching to and from another input method very often.
But I also think that it's easy to argue the point that the device designed with the sole purpose of entering text is not optimal for navigating characters in 2D/3D space. Something better just hasn't come along yet or it hasn't been popularized enough. An improved mouse design could also offload a lot of inputs from the kb hand, like those MMORPG mice with macro buttons. There's definitely room for improvement.
Notice, that I didn't say it's impossible to use KB for gaming, I just said (or implied) that it's not optimal. It's not good, it's just what we have and what we are used to, and there's a lot of room for improvement. And I say this as a PC gamer of 30+ years. So much so that when I lay my hand on a keyboard it automatically assumes the WASD position, even if I intend to type.
And I'm glad that I brought this perspective to your attention, because we often don't question what we are used to since childhood. Hopefully, these discussions will eventually bring us something better, that we have not yet thought of.