Because unlike those assassins veigar is not mobile, needs a long time to get to that point and has large cooldowns that mean if he misses something there's lots of time for counter play.
Lets take pre nerf talon (lets go back a few sets of nerfs here, not just the most recent ones) and compare it to veigar.
Veigar is ahead and wants to fight. He tries to E. That E can be dodged by talon by walking, flashing or using his gapcloser on a minion or enemy champion. Veigar can ult but he's vs a target with no AP meaning unless we're talking 40+ minutes into to the game he's not gonna 1 shot him.
Talon is ahead and wants to fight. He jumps in, autos, Q's, ults and uses W (they cast at same time) and uses tiamat. Congratz he just did more than double your health while you were silenced. None of the abilities used could have been stopped, dodged or countered because you were silenced and the entire combo took under 1 second.
Hell pre-pre-nerf talon had way more dmg as well so he could kill veigar just by jumping in, using AA, Q, W. He still has ignite, flash, ult and hydra active if needed to jump onto secondary targets or stealth away.
#counterplay