Whey do non-catholics who have been baptized in another recognized Christain faith such as “Lutheran” have to be re-baptized when going through the RCIA process?
I think you have some information mixed up.
If someone was baptized Lutheran (as in your example) they would not be re-baptized in the Catholic Church. Lutheran baptisms are considered valid. The theology of baptism does not require an ordained minister for validity. Anyone can baptize as long as they have the right matter (water), form (baptize in the name of the Trinity), and intention (they intend to baptize). Most mainstream Protestant denominations have valid baptisms. For a more extensive list here’s one from the Archdiocese of Baltimore.
Your thread title, however, asks about confirmation. In order for the sacrament of confirmation to be valid a validly ordained minister **is **required. Therefore only those Christian communities that are Churches with validly ordained Bishops and Priests would have valid confirmations. That link above from the Archdiocese of Baltimore also lists which denominations have valid confirmations. Using your example of a Lutheran confirmation, their ordinations are not considered valid, therefore they do not have validly ordained clergy and thus do not have valid confirmations.