Run the code in SAS
data _null_;
if 1 or sum(.) then put 'Hi!';
run;
It reports missing value was generated which shows 'or' is not a short circuit operator.
Why doesn't SAS support short circuit operation?
The strongest argument is for continued proper operation of all legacy SAS code, that knowingly or unknowingly rely on it.
As for the original why, that would be a question for the originators. https://en.wikipedia.org/wiki/SAS_(software)#Origins
There may be un-documented options for causing the DATA Step compiler to generate 'optimized' code that might include shortcut evaluations.