Because while yes it is kinda like terrain it is actually supposed to consistently push back enemy champions that try to dash through it (tosses them flying through the air back to the front of the wall). Fizz is the only champion who can hop the wall besides Vi, Jarvan and Malphite due to spell immunity during their ults (Olaf too? Idk).
I mean, if this is how it's meant to be then it's no problem I'll just have to better myself as a player and find a way to fight him with this fact in mind. But if it's a glitch/bug then it'd save me a whole lot of heartache in this matchup.