[James2001]

And besides, they have to film future episodes based on the ending that's going to be used anyway, it's not like they can film multiple different versions of several weeks worth of episodes to help keep the secret.

Though I guess when, for example, it's a trial, and there's going to be cast and crew filming those scenes who won't be involved in the day to day production of the soap, filming multiple endings helps keep it secret from them.