It's a bit like asking why Google doesn't just partner with Apple on phones since Apple is clearly better at it. The reason is that they are building competing

It's a bit like asking why Google doesn't just partner with Apple on phones since Apple is clearly better at it. The reason is that they are building competing products, with competing teams, competing software stacks, and different business models. Competition is good for innovation. Tesla prioritized building a highly generalized stack that can offer incremental improvements from L2+ on all roads (today) to L3/L4 some time in the future. Waymo is prioritizing building a tightly geofenced L4 system and gradually expanding the operating domain. Neither is more of a "correct" approach, since they have different design goals, and it's hard to even directly compare them at this stage of development.