I do understand where you are coming from. Without disagreeing with you or indeed expressing an opinion on the issue one way or the other, for I no longer really have one, I can offer this explanation: it tends to be the case that in most Congregational churches, including UCC churches, at least when I worked for them, the actual voting members represent what is in many cases only small subset of the congregation, with membership being optional in terms of partaking of the Eucharist, and many other aspects of church life, and entailing additional responsibilities, so when someone new makes the commitment to formally join as an actual member, it is kind of a big deal, which does bear some liturgical similarity to Confirmation or the Orthodox service of Chrismation.