If I were to guess, it would be that the predictors have a very small effect, relative to the constant term in the model. Study the following simulated data. The explanatory makes a relatively small contribution to the linear model. Even though x variable is significant (small p-value), the variable just doesn't have much of an effect. The predicted probabilities are all less than 0.5. data a;
call streaminit(1234);
do i = 1 to 1000;
x = rand("normal");
eta = -1 + 0.15*x;
y = rand("bernoulli", logistic(eta));
output;
end;
run;
proc logist data=a plots(only)=fitplot;
model y(event='1') = x;
run;
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