Because a nuke detonated at ground level is not worth much. To really get the effect you gotta have it up in the air a few thousand feet.
It definitely helps if it’s up there a ways, but if you intentionally build it to be dirty it’s still going to kill a lot of people and keep on killing for a long time to come.
If you want to scare the bejeebers out of yourself read One Seacond After by William Forstchen. The premise is two nukes launched in the Gulf of Mexico from container ships for an EMP strike. I’ve yet to find anyone who can calm my fears about this scenario.