Why no kevlar vest?

I saw the police patrolling public places like MRT.It's good that they provide a sense of security on public places for the citizens.

But my question is,why no kevlar vest?Or do we assume the water is safe,we do not need life-vest?We assume,that nothing will ever happen?We assume that possible attackers will only carry plastic for & spoons to attack & we can subdue the attacker with unarmed combat?