The king was very careful to ensure all the gatekeepers were following the process correctly. It did not occur to anybody that one of the gatekeepers could be using a dirty rag to jam their gate closed instead of a lock.
Once discovered, the wall could be trivially bypassed. The security of the whole internet was immediately broken. Rebuilding the wall to block up the dodgy gate took much effort from very many people.
This system was invented in the 90s when things were looking up. China was opening up to the world, the Russians were suddenly not evil anymore, and the internet was supposed to bring us all together into one happy global society, kumbaya and all that. Obviously that didn't work out.
It's time for a rethink really. Every CA is now a single point of failure.