Zed is a manaless champion who can spam his skills to simultaneously free farm an poke you without any consequence. Whenever Zed commits to a kill, he's not committing his safety, just his damage. When Fizz goes all in, he ends up "in" and can't get out if he used his troll pole to do the damage. Zed on the other hand just goes in, dishes out all his damage in a second, and gets back out without worry of retaliation. Fizz has to choose intelligent times to attack a squishy target cause punishment is possible. Zed just has to have his ult up and remember to hit R again when he's done.
Comparing Zed and Ekko is a bit weird. While they were both designed to be assassins, Ekko has become an AP bruiser and can no longer be compared to Zed directly with assassin reasoning.