Why there is no brush library in Inkscape?

Because Inkscape manipulates vectors, not individual pixels. The "brush" metaphor doesn't make sense in this environment.

https://www.adobe.com/ie/creativecloud/file-types/image/comparison/raster-vs-vector.html

You can however simulate this behaviour using the Spray tool to splatter the canvas with multiple copies of a shape.

The following topic I shared many months ago has some interesting shapes that can be used in Inkscape. There are in the attached svg files.

The link is:

https://inkscape.org/forums/tutorials/examples-of-brush-shapes-i-made/