Because they are not baptized, they are not in the Church, and if they reach the age of reason they go to hell. Limbo is actually God's mercy. You see limbo as a punishment because you assume that people who are not Catholics can save themselves by being nice.
They can't save themselves even if they shed their blood for Christ, how much clearer can dogma make it?
Perhaps I should've looked more carefully to determine *what* infants we were talking about. The Holy Innocents were killed for the faith, because herod wanted to kill Christ. And they were in Israel, which was the "church" of the Old Testament. It sure seems to me that hypothetically speaking, an unbaptized child of a Catholic parent who was martyred (say ISIS killed the whole family for being Catholics) would be equivalent to something like that, and that seems like a pretty reasonable application of Baptism of *Blood*. Now if you wanna say that the child of some random infant who dies (with their death having nothing to do with the faith, not being for the faith even in an indirect sense) it seems less likely BOB would apply, and more likely they'd end up in Limbo. But to say that children who die under equivalent circuмstances to the Holy Innocents end up in Limbo because of some arbitrary legal change from God seems more like Protestant style nominalism where God arbitrarily decides whether to impute guilt or not than it does the logic of Catholicism/