Veigar does not struggle because of mobility creep. Of nearly all the immobile mages in the game, he probably suffers the least specifically due to mobility, because a) 80% of his burst is targeted spells and b) he has a huge ass cage that only champions with blinks can go through without getting stunned. You don't even have to be close to accurate with the stun, so long as a mobile champion is inside the cage, they can't get out without getting stunned unless they flash or have a built in blink, which few champions do. That gives Veigar much more time to escape than most mages get.
He suffers because his early game is worth fuck all. Lux, an immobile mage, will give Veigar a very hard laning phase. Ziggs? Same deal. Malzahar? Yup, wrecks Veigar. Xerath? Same thing. He suffers because his late game, just like most burst mages, is shit because banshees and QSS destroy his ability to gib, as well as the fact that he can't get to the backline to actually delete squishies. This gives him one stage in the game where he can shine - the mid game - where he can catch people out and will be able to delete squishies because they don't have defensive items yet. His mid game is shot if his laning phase goes extremely poorly - and look at that, his laning sucks and he loses to pretty much everyone that isn't Karthus.
If Veigar is doing well, dashes mean shit to him 1 v 1. Blinks still matter, but if Veigar is fed? Ahri's R isn't gonna save her when he drops the cage and then deletes her with his combo.