Vectors have to be rasterized at some point, which requires detailed knowledge of device pixels.
Furthermore, some graphics are always going to be bitmaps, such as photos. Tasks like "center this image square, but don't make it blurry" requires pixel-perfect positioning.
That's exactly my point. Only the mobile OS knows the detailed knowledge of device pixels, not a web developer. Raster graphics can utilize GPU accelerated filtering. Sharp graphics should be vector with blocky or any other kind of 'hinting' that you might want.
Furthermore, some graphics are always going to be bitmaps, such as photos. Tasks like "center this image square, but don't make it blurry" requires pixel-perfect positioning.