You sure about that?
That's the way offside calls are supposed to work. But for penalties the process is based off the idea of there being a clear and obvious error with the referee's initial decision. In which case not calling the penalty becomes as firm a decision as calling it.
VAR can't just call him back to review it as they would with an offside (which is an objective decision), they can only call him back if he's made a definite mistake. So if he's 70% sure it's a penalty but decides not to give it, the penalty then can only be overturned if VAR are 95% sure it isn't one. The referee can't just say "I'm not sure" and leave it up to VAR, he has to make a call and that call carries heavy weight. So he should just be making the call he thinks is correct, not letting it slide if he thinks it's a penalty because VAR will catch it.