Burst champs have something gating their burst.
Annie, Veigar, Syndar have substantial cooldowns on their ultimates (which they rely on to burst). Akali and Diana have to dive in, and don't have a way out. Etc.
Zoe has no significant gates on her combo. Her Q, E, and R all have low cooldowns. She is ultra long ranged, and does not have to commit to anything to be able to drop her entire combo (or, at least, her entire combo minus passive). She can basically stand behind a wall and chuck out her combo on cooldown.
Zoe should not be a burst champion. If she is a burst champion, that is a problem IMO. Her play pattern is that of a poke champion. She should be balanced as a poke champion.
Burst champions don't burst from a screen away every 7 seconds without ever having to commit. That's a poke pattern. She's a poke champion.