My immediate thought: The arrival of the Christ was a momentous, capstone event. For those who were already part of the (pre-Christ's-arrival) visible church at the time, Christ's arrival was the fulfillment of the whole reason that church existed and the whole purpose behind its worship. So, you can be a perfectly upstanding member of that visible church (like Zechariah in Luke 1:6), but if you fail to believe the gospel of Jesus (Luke 1:19-20) and reject his visit from on high (Luke 1:78-79), the church will be moving forward without you. Christ's arrival is that pivotal.
Therefore, it makes sense that to remain in the visible church you need to confess Christ and believe the gospel—and receive the new sign of admission to the church that connects you to Christ by name and recognizes your alignment with him. In a sense you are only continuing a visible alignment with Christ you already had as part of the pre-Christ's-arrival church. We recognize that. But we also should see that this new age comes with a new, fullness-of-time sign of membership in Christ, so that for those particular believers at that particular time there's also a sense of readmission or continued admission. They too can feel brought in or added (Acts 2:41, 47).
Don't get too stuck on the question, "If they were already in, how can their baptism mean they are getting in?" Suppose you were subject to a king who built a temporary castle to house his people, with the goal of one day moving to a larger and more inclusive castle once it was built. When that new castle is ready and the moving day comes, you who were already part of the kingdom need to follow your king into the new castle where his kingdom will continue and expand. You will go in again, through the new and wider gates, along with everyone who is entering for the first time. You were already in, but you are also newly in, because the momentous event you were waiting for has just occurred.