Because it doesn't take a genius to know when the jig is up. By the 20's Poland had already put the squeeze on Jews, forcing them from their homes. Hitler didn't outlaw his first bagel until after '33. Your second question belies the first, but were I to guess, it's because the Jews in Poland were more loyal to their country than their country was to THEM.
What part of "young, able-bodied, trained to use arms" do you not understand? These young people were such ardent Zionists that they knowingly and willingly abandoned those who needed them most, at precisely the moment they were needed the most (i.e. not in the twenties, but right before the outbreak of WW2). They could have helped the Poles fight Germans, but didn't think it necessary.
Therefore, my second question does not belie the first.
Also, I want hard evidence for the Polish government "forcing Jews from their homes" in the twenties, or the thirties, for that matter. Can you give it? I think not.