Beef that's ground at the processing plant is often contaminated and can't realistically be cleaned, whereas flat cuts and roasts can be rinsed off before packaging. Beef that's ground at the supermarket is often less contaminated because there aren't other bits of the animal in close proximity at that point. The cleanliness of the supermarket's meat department still matters a lot, though.
Beyond that, if only the exterior of the hunk of meat is contaminated, then only the exterior needs to be heated to a high temperature. If you sear a steak on all sides, the interior can probably be pink and cool and it's OK. Not so with a hamburger -- it would have to be seared all the way through.
This is basically why the FDA wanted to start irradiating ground beef after it was packaged. It's not the worst policy idea they ever had. Personally, I consider raw meat a survival food, and some people find it, uh, difficult to digest, which is part of why it's usually cut into small pieces.
