It’s because it’s so basic. People don’t tend to make stock of things that are faster to create from scratch than to upload or find online xP
If you want a solid background, you can set the Background Color property on the map itself. It doesn’t really make sense to do it with tiles xP You could also just leave it as-is, as the black colour is usually just the default fill colour used when rendering the map at runtime. Of course, setting it in Tiled will give you a more accurate preview.