Why is Trump not firing these people?
It would also be easy to find out who is leaking as well. You tell people individual whatever you want kept secret and make different enough. You do this in writing and verbally. When it is reported in the news media it would be easy to trace back.
The washington post revealed telephone transcripts of private conversations from the president. Arrest the reporters and editors and the owner. That would stop the leaks and reveal who gave them the transcripts.
Why is Trump not firing these people?
I’m afraid Trump is just not very interested in diving in the weeds on foreign policy. He’s was a businessman and focused on domestic issues as much as possible.
WaPo has an article today, (thread upstream) where it publishes transcripts of Trump phone calls to Mexico and Australia. From the WaPo article
The Post is publishing reproductions rather than original documents in order to protect sources. The reproductions below also include minor spelling and grammatical mistakes that appeared in the documents.