TLDR: You can't really, but you can kind of*
In Inkscape lines, or shapes (except for rectangles**), are generally constructed using the concept of vector paths, to which fills and strokes are applied. That's how the software works. There's no Line Segment Tool in Inkscape, like the one in Illustrator.
*It is possible to link or embed raster images in Inkscape and other vector image editors, but these will remain as raster images made of pixels. It's also possible to rasterize objects made in Inkscape using Edit > Make Bitmap Copy, but again these will then be made of pixels, and not vector.
*Also, you could use underscore characters to make a line, which would of course be a text object, rather than a path. But then again, strictly speaking, fonts are still made of vector paths.
**Also possible is to create a long rectangle, and fill it. Technically not a path as such, but an SVG rect object, as you will see if you examine the XML code.
Edit: further to the comments with user287001, it would seem there are other possibilities. Illustrator has a Line Segment Tool, and a line made with it, and exported as SVG, will create an SVG line. Another possibility is to hand code the line segment. Shouldn't be too hard to hack it if you can find some examples. And strangely enough Inkscape can see these SVG elements if you open such an SVG in Inkscape.