Because if given enough time, I can easily see him 1 shotting any champion, with any build.
I bet if he has enough stacks, his burst will actually happen so fast and with such great force, that 
and 
will literally be destroyed on impact and will have to be repurchased. All while at the same time scaring Baron back into his hole for the remainder of the match where he will passively just buff Veigar indefinitely.
paperbIade4 votesno you idiot if he reaches late game he deserves to one shot you
deoje2992 votesThis is true but the games rarely last long enough for that.
JavelinJoe1 votesyes, but his W and R dealing 1-1.5k magic damage isn't as rare of an occurrence sadly.
deoje2991 votesYes, but he also is immobile and extremely squishy.
Skelenth2 votesSo would Nasus and Thresh, given time. And in the end, Sion would be the only one who could tank them. But before Veigar reaches 3000 ap, the game will be over.
JavelinJoe1 votesyep, but Nasus is melee only for him to use the stack damage, and they do not get boosted by any other items (unlike how veigar's stacks are directly added to his AP and able to be boosted by DFC). Same as thresh, he has to hope a sole drops to get them and really cant run full AP with as much success.
A Dizzy Lulu1 voteshuh
reaperteaser1 votesand if he has enough stacks he can 1 shot towers using lichbane
MrHaZeYo1 votesIf you let Veigs get that far your doing something wrong, hes vulnerable in lane, just like Nasus except he isn't tanky AF to help him out. He does have his event horizon but its easy to at least not get CC'd and dodge the slow animation of his W.
Furthermore, he's a burst/battle mage, he does what hes intended to do. If he gets enough stacks to blow through a combo you most be at like 60mins in the game, which by then he again should be able to do what he does.