To add to this, the earth is also frequently not a large enough capacitor, especially for higher power applications. Large radio stations and radar stations often have a metal mesh placed on or under the ground nearby to make the “earth” a better capacitor. Lightning discharge spikes often extend deep into the foundations of buildings, or sometimes have a spike that extends below the foundations of the building, to get a better ground connection.

So the earth is often assumed to be infinite during the initial design stage, and if it turns out not to be “infinite” enough we can supplement the “earth” so that our design still works.