I can make a case: Building their own models like Nova and Titan allow them to build up expertise in how to solve hyperscaler problems. Think of it like Aurora, where they have a generally solved problem (RDBMS) but it needs to be modified to work with the existing low-level primitives. Yes, it can be done in the open, but if I'm AWS, I probably want to jealously guard anything that could be a key differentiator.