Because arming guards who are in crowds is dangerous.
Well, almost.
A guard drawing his gun in a crowd is dangerous.
Consider coming out of a Walmart cash office a week before Christmas with their weekend receipts in your arms, all by yourself pushing through a crowd packed shoulder to shoulder.
Could I pull my sidearm in that situation?
Obviously not.
Not unless I could deliver a contact shot to an attacker's body, not bloody likely.
You know what didn't happen?
They didn't take my gun away.
They don't take them from from the PM's security detail either.
Also obviously, he is not always in a crowd.
Give your head a shake.