Part of what marks it as a code share is those marketing carrier/flight and operating carrier/flight fields that get propagated out lots of places internally and externally. If you just up and change those, then your flight listings would look wonky on, say Expedia. Or your reconciliation process to get the revenue in the right place might break, etc.

So the ego thing is somewhat true, but that decision happened long ago, and already created tendrils that are hard to unwind.