How so?
Because their core beliefs are those of the Church which they took from the church when they left her.
They did not redefine Christianity, they rejected certain aspects of the faith of the Church.
Exactly what cafeteria Catholics are said to do only to the extreme indeed.
We "took" what we believe from Holy Scripture which combines the teachings of Jesus as well as God's subsequent revelation to the others to whom he utilized to create what we call the Bible. Roman Catholics also derived some of their core doctrines from the same source, so it is hardly accurate to insist we "took" anything from Catholicism.
True, we did not redefine Christianity, but the Catholic Church had and where their dogmas contradict Scripture, we reject the most grievous ones. Remember, the intent of the Reformation was NOT to break apart the church, but to steer it back to original orthodoxy. Those in charge at the time loved their wealth and power too much so a break had to happen.