The example of singularity provides 12% more hit damage to all hits. Crit as a baseline offers 5% chance of 200% damage, which is an average of 5% more damage, however far less reliable in when you actually get more damage. Crit can certainly be scaled higher than what singularity can offer.
However singularity is intended to fill a place for builds that may use slower more powerful attacks where having unreliable crit chance can be unfriendly, or for hybrid hit/dot builds which don’t want to invest passives and affixes into crit. Or even to run until you get to a point that your crit can out-scale the damage singularity offers.
It’s not designed to be a be-all end-all damage boost to all builds, but provide a boost to certain builds in leu of stacking crit affixes for the cost of a single 1x1 idol slot.