What graphics file format?
If EPS or PDF, the answer is: because computers and graphics used to be too slow (we're talkin' 1993 here).
With some image formats, like EPS or PDF, what you see in FM during edit is a 72 dpi preview or thumbnail image, basically a GIF, often not even dithered. What you render to a PostScript workflow (including PDF) is the original EPS data (and PDF is treated as EPS).
Now that we're in the 21st century, it would be nice if FM had an option for displaying the full image. It doesn't, so if the coarse preview becomes a problem, there are work-arounds, like the EPS scaling trick.