For the same reason credobaptists take their babies to church and read to them from picture Bibles. Because they're born with a sin nature in a corrupted world and we choose not to withhold any means by which God establishes His relationship with people. In short, our practice derives its template from a 1 Cor 10:1-4 paradigm. Like Paul, we assume that the practice of "all" didn't change. We also assert with Paul that entrance into the life of the church does not necessitate continued faithfulness.
So for your church it is like circumcision.