If you’re looking for answers from Protestants, might want to hop over to Christian forums and ask there.
But, from a Catholic perspective, baptism does bring us all into the Body of Christ (provided it is properly done, as one poster also mentioned). That can never be undone and never needs to be done again. In this way, a properly baptised Protestant is already a Catholic - because he is part of the living, breathing Body, which is the Church.
However, the Apostolic Churches - the Catholic and Orthodox - will not (and should not) admit anyone to communion or any other sacraments that will not recognise the authority of the bishops (and in the case of the Catholic Church, the Pope) of that communion.
Now some people call this “political”, but the reason for this is simple: Christ gave authority to His Apostles to teach and discipline His Church. (This is in Scripture. It is also in Scripture that the Apostles appointed successor “bishops”, or overseers.) He also gave them the ability to confect and perform the sacraments.
Now all of the Sacraments are a sign that you wish to follow not only Christ and the Bible, but His Church and His Apostles, because you believe them to be true authorities. So how can a Protestant, who does not recognise the genuine authority of our bishops and their priests, come to receive that which he does not believe in?
I don’t know why Protestants think authority does not matter. They used to, centuries ago. Closed communion existed for a reason, and that was because communion (although not a sacrament among Protestants) is a sign that you not only follow Jesus Christ, but you follow Him as the church teaches - if you are a Presbyterian, you follow Jesus as the Presbyterian church teaches. If you are a Lutheran, Missouri Synod, you follow him as that Church teaches. It would be a mistake for a Lutheran, a Calvinist, and a Pentecostal to all proclaim they follow Jesus in largely the same way.