I'm not sure how folks write in the same genre all the time. I think I would get so bored. On the other hand, I do only write romance so maybe I can kinda see this. But within the romance broad category there are so many different flavors. I've written fantasy, paranormal time travels, futuristic sci-fi stories and historical westerns. Right now I'm bouncing between a historical fantasy and a historical western.
At first I thought, wow, how odd to be going back and forth but then when you think about it, a lot of fantasy books have the same qualities as a western. You have your host of bad guys all harmed to the hilt in some way. Cowboys have their guns and trolls have their sharp teeth and claws. You have the damsel in distress. She's your cowgirl who needs a hero but can still handle herself when it all falls apart. The heroine in my fantasy is pretty much the same way. She needs the hero and yet she's leading him around and pretty much showing him how much he needs her. Then you have the sexy hero. He's a cowboy through and through. A quiet man with strong morals and a belief in justice. He's strong willed, an alpha with beta tendencies and while he has his faults, he always manages to save the day. My hero in the fantasy has all these qualities and more.
I remember when Star Wars first came out in theaters. My then boyfriend took me to see it seven times. We were captivated. Critics claimed it was a western set in outer space. And yep, that pretty much sums it up. With that description, it's easy to see how a sci-fi writer can find herself penning historical westerns as well.