I didn’t actually. But I do agree that historically Lutherans have used the confessions as guiding and authoritative interpretations of Scripture, which is the normative rule of faith and life. However, as we all know, some Lutheran denominations have more stricter conformity to the Confessions than others.
They don’t have the right of private interpretation on issues that their church closes off to debate, which is the same situation in any other church including the Catholic Church. Some churches give more room for private interpretation than others. You have to look at each denomination on its own merit. You can’t just make blank statements such as “Protestants don’t believe in sin because they have private interpretation.”
The point I’m making is that there are different Lutheran hierarchies doing the interpreting. Each Lutheran denomination has its own hierarchy.
As to why there are different Lutheran denominations in America, that stems from the US having its origins as a British colony. Lutherans came to the New World from a variety of nations. There were German Lutherans (and each German territory had its own territorial Lutheran church) and there were Scandinavian Lutherans (again each coming from different national churches). The Lutheran denominations we have today form out of the various mergers of this hodgepodge of immigrant churches.
And yes, certain groups of Lutherans had differing interpretations of the Lutheran Confessions and the Scripture. So there were ethnic, cultural, and doctrinal reasons for the multiplicity of Lutheran denominations that originated in America.
Just to use an example, the ELCA itself has only existed since 1988 when 3 Lutheran churches merged. One of those churches was the American Lutheran Church, which itself had existed since 1960, resulted from the merger of 3 older churches. One of these was the Norwegian Lutheran Church of America founded in 1917.
This Norwegian Lutheran church was itself a merger of the Hauge Synod, Norwegian Synod, and the United Norwegian Lutheran Church of America.
This seems chaotic, but its not surprising. When the first Lutherans got to America, there was no Lutheran churches or hierarchies or institutions. They had to build these, and different Lutherans built their own institutions among their own ethnic groups.