Veigar punishes people for playing well
Oh, you somehow managed to beat me in lane? Killed me a few times? Well it doesn't matter because I outscale you and my ult scales off YOUR AP! And if you're not AP the burst is high enough that it instapops you anyways. So if anyone on your team did well (your adc for instance), they immediately become irrelevant in teamfights. And the stun is super easy to land. And if he somehow misses he'll just flash for you.