Boeing outsourced the fuselage to Boeing in Wichita, which had always built them. Boeing spun off that branch into a separate company now called Spirit, which is still basically Boeing except for its name.
Spirit is not just Boeing. Since the Spin-out Spirit has been buying many other companies and they are a supplier to both Boeing and Airbus. About 65% of their costumer base is Boeing. Spirit is a totally independent company.
If it's "still Boeing except for the name", that still smells funny.
They got something out of spawning a new corporate entity. Without a convincing TL;DR I think most of us will assume it's for reasons that sound good to Boeing business people, but not to outside people. (I. e. being a "new entity" to avoid existing labour contracts, or serving as a liability firewall in the event chunks of plane start flying off)