[2023] Castle Sieges: Why Instanced?

Castle sieges, as far as I am aware, are set to be 250v250 with the hopes to expand upwards of 500v500.

My question is simple, with these colossal numbers why even bother instances them in the first place? Strict sides remove alot of emergent player behavior and politics behind the sieges such as 3rd parties.

No 3rd parties no last moment betrayals no " look to the west on the third day" moments. All of these will be seemingly possible le during node sieges so why differentiate castle sieges?