Because it isn't. PoS suffers from a "nothing at risk" problem.  An attacker can use coins they no longer have but did have at one point in the past to attack the network.  Actually there is no reason to not do this.  If the attacker is unsucessful well they lose nothing in the attempt and if they are successful they get all their coins "back" (that they may have lost, had stolen, or sold).  Checkpoints limit how far back the chain can be reorged but they don't solve the nothing at stake problem, only limit the extent of the damage.