In the 2018 movie Replicas with Keanu Reeves, why didn't he simply tell his family what happened once they had woken up, and say he had to clone them to save their lives? Then he could use one of the now-empty cloning pods to save Zoe as well, eliminating the need to erase Zoe from their minds and keep up the appearance around the house that Zoe never existed (i.e. her memorabilia) and that 20 days hadn't passed. (i.e. the spoiled milk.)
William Foster, doesn't simply tell his family what happened because he knows that his actions were illegal and unethical. Cloning humans is illegal in the movie context.
As for saving Zoe, using an empty cloning pod would not have been possible as the pods require a complete set of DNA in order to work, and without Zoe's original body, it would have been impossible to create a clone of her.
William's decision to erase Zoe's memories and keep up the appearance around the house that she never existed was a way to protect her and the rest of his family from the consequences of his illegal and unethical actions. While his choices were controversial, he believed they were necessary to save the people he loved.
Declaration: I've not seen the film myself, and having read the reviews, I'm unlikely to attempt to do so with any degree of urgency.
- It is generaly regarded as being badly structured and not logicaly self-consistent:
Variety's Joe Leydon criticized the film for its "cavernous plot holes, risible dialogue, and ludicrously illogical behavior"
- Rotten tomatoes has it:
Critics Consensus: Equal parts plot holes and unintentional laughs ...
- Charles Bramesco of The Guardian:
Reeves has figured out how to adapt to the [.....] gaps in logic like a genetically altered fish to water.
As to a direct answer to the question:
Why couldn't William save Zoe too?
I'd suggest it's a set of choices by:
Screenwriter - Chad St. John
Director - Jeffrey Nachmanoff
On this occasion, there was no direct studio pressure breathing down the back of the filmmakers' necks to make a particular set of choices, as it was taken up by Entertainment Studios only after the film was complete.