Because that wasn't part of the treaty man
Anglo-Polish Military Alliance
In response to Nazi Germany’s occupation of Czechoslovakia, Britain and France pledged their support to Poland guaranteeing its independence in the face of Nazi aggression.
The United Kingdom, sensing a trend of German expansionism, sought to discourage German aggression by this show of solidarity. In a secret protocol of the pact, the United Kingdom offered assistance in the case of an attack on Poland specifically by Germany,[3] but in the case of attack by other countries, the parties were required only to "consult together on measures to be taken in common".[13]
Exactly. This further goes to show that Britain and France picked a fight with Germany. Germany and Poland were having a dispute and Britain decided to get involved as means to confront Germany. They didn't form their alliance until after Germany and Poland were already in dispute. They did this to provide a pathway for war.
They weren't concerned about Polands safety or autonomy, they didn't have any moral concerns, and they weren't being threatened by Germany. If they were concerned about these things then they would have confronted Communist expansion into Poland as well. And this all goes to show that Churchill and Co were to blame for starting the war. Germany made many appeals for peace and were rejected every single time.