Nah, you're not alone. RPing and writing borrow a lot of the same skills and have some overlap in my brain but RPing tires me out like ten times faster. It's that "you're in the character's present" feel, and that you're viewing the world strictly through their lens, and no you have idea what your RP partner's going to throw at you. I really enjoy that cooperative writing feeling.

But I also like the "having control over all the aspects of a situation" that writing offers. You have the room to tweak it endlessly and the time to re-think things when they aren't working out. I can explore what-ifs to their conclusions in the knowledge that I'm doing it on my own time, without dragging somebody who may or may not enjoy it with me. And that's enormously freeing.