They don't need to. f2.8 is wide enough for good auto/manual focus, and the depth of field is about 5mm or so wide open (or so my measuring tape tells me).

On the other hand, the 60mm EF-S I have here goes as high as f32, for increased depth of field. And it's pretty darned sharp most of the way there.

Note that the 60mm is equivalent to 96mm on my Canon 40D, so it is effectively a 100mm macro lens.