Why's viegars ultimate targeted and not a skillshot? Curiousity

Most (projectile [i feel odd saying this]) ultimates (to my knowledge that do about or more base damage than viegars ultimate are skillshots.) rivens ult does quite a bit of damage as she scales throughout the game, ekkos is somewhat of a skillshot and I'm sure we've all seen how much damage release ekkos ultimate did with how awkward it can be to time/land, Aurelion Sol's ultimate did ALOT more damage when he was released, If we count nidalees spear which punishes players for not standing behind minions, and im sure there are other champions that I've forgotten to meantion. Personally I don't really like fighting or playing against him mainly because of how much damage he does (he has to earn it but slightly faster than nasus.) My feelings aside, this is just a question i ask myself occasionally. Is it because the skill is an execution type ability? I've generally tried to compare his ultimate to Fizz's (pre-nerf to post) and Viegars. Fizz originally just needed to land the skillshot to get the massive damge he wanted but is now rewarded for landing it at greater distances and realistically can do a tremendous amount of damage to a target with a low overall health pool. With viegars he realistically needs about a good 15 minutes of farm or 10- 15 with a good amount of kills to make you respect his massive damage which continues to scale if he is allowed to farm (lets be honest you cant babysit him forever.) He isn't all that difficult for me to fight, I'm just a bit curious. TL;DR Title.