If you have e.g. a diagonal line from a vector graphic then it will almost always be antialiased (i.e. light grey pixels along the edge of the black pixels - or maybe there aren't even any black pixels and they're just dark grey, which might look fuzzy to some).
The alternative is that you convert the vector line with a "nearest neighbour" pixel algorithm (or cleverer but fundamentally similar algorithms e.g. [1]) but then you get the aliasing artefacts that anti-aliasing is meant to avoid. For example, two parallel lines that are rendered 2.5 pixels apart might end up having a gap between them that alternates between 1 and 2 pixels along its length, which looks even worse.
[1] https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
With hand-designed pixel graphics, this isn't a problem because you can manually choose to draw everything an integer number of pixels apart or avoid certain constructions altogether.