He said transistors, not IC's. And electromigration is a problem with metal interconnects at high current densities. So you talking solemnly about a failure type that can occur with on some integrated circuits under some conditions, or when the designers screwed up. But which doesn't actually happen much in practice.
The persons actually question is roughly why do transistors last so long compared to other types of mechanisms. No one in the comments made an attempt to answer that, at all.
