There are lots, it's just that many mostly advertise themselves as component libraries for particular frameworks. Oftentimes you'll find that the common core is just plain ol' CSS with a little bit of JS in the background.
- I want something lightweight and not bother to deal with compiling/preprocessing (like Milligram).
- I want a MVP without classes (like Picnic.css)
Otherwise I pay a premium theme or create one myself.