Ah, I guess that's where we differ. These days I'm no so interested in being a writer as much as I just want to get the ideas in my head out and make them exist. Basically, as @Caro put it, I want to "just perfect a set of ideas, concepts, and themes". If I get those ideas down within my lifetime and never have anything new pique my interest ever again, I'm happy to set down my writers tools for the rest of my life 
Most people here seem to be interested in about 3-17 different themes. Is that enough for ya? 
Jokes aside, it's generally not feasible to just one day decide "oh, I'm going to be interested in that theme". Interest is an emotion, and it's not something we can control. Granted, some people do get 'stuck' and have to go out of their way to seek out new things in order to broaden their interests, but even for them, not everything they seek out is going to 'stick' and become something they're genuinely interested in.
For me personally, this 'shotgun approach' isn't very efficient; I'd say I probably have less than 10% success rate for acquiring new interests just by seeking out random new things, and thus more than 90% of the time I spent feels like a waste. I find it more efficient to branch out from my existing interests, because stuff that's adjacent to my current interests are more likely to interest me as well.
An artist who draws cars all the time might play around with the designs for the frame, which leads them to looking at frames of other vehicles like boats and planes, perhaps just for inspiration at first, but then develop a full fledged interest in boats or planes and start drawing them as well.
And so people's interests are going to shift over time anyway, even without actively seeking out new things! The world is changing around us, and we ourselves get bored when we explore all there is to explore of our old interests, and of course we're not going to force ourselves to keep sticking to those old interests if even we're bored of them! And conversely, if we naturally feel like sticking with the same themes we know and love, it probably means there's more to be explored. (That, or market forces have pigeonholed us into a niche and it's others who are pressuring us to stick to the old themes, but we can worry about that when we become famous XD)
(emphasis mine)
So then, if only a fraction of the theme is needed to tell a story, what do you do if you want to explore everything else about the theme? Make a new story, probably
(Of course, I think it's optimal to merge your stories so that you have a robust enough story to support everything about the theme, but you also don't want to shoehorn irrelevant aspects of a theme into a story that would be tighter without it, so half-explored themes might be kind of inevitable if you care about the structural integrity of your story XD)
So I guess I can see why diversification would be important if you're a career writer who needs to put out a large volume of work (and indeed are even capable of producing a large volume of work since you're only responsible for the writing and have a team to handle everything else like art etc.) Sometimes forcing yourself to broaden your horizons might be necessary to keep your work from being repetitive.
But for independent creators who do everything themselves (and plan to stay that way), I feel like our output rate is so slow that once we finish a work, our interests would've naturally shifted enough such that our next work is going to be different whether we like it or not 