Tournament of Ages is also something Blizzard has spotlighted multiple times, and it wouldn’t be able to exist with sharding on RP servers.
During prepatch they turned sharding off for a few days and it allowed for a huge server event in Stormwind, dealing with an elemental attack. It was laggy, yes, but it was very fun. Some of the fondest memories players have, not even just RPers, are of huge events that either didn’t have much sharding or had so many people it didn’t matter.
The Legion invasions in the Legion prepatch, for example, huge lag fests. But that’s still regarded as the best prepatch ever.