> why is one physical iPhone identical to another physical iPhone
Apple is willing to pay the cost to minimize the margin-of-error at every step.
Another approach to keeping yield high (at the possible risk of hurting your brand) is to do what Intel did with their Celeron line. The equivalent for Apple would be a "value iPhone" which would not come with the same set of expectations from customers.
It's worth noting Apple (likely) do this with the SOCs - the bucketed variants with fewer operational GPU cores are the output. They just don't do it with the packaging.
Apple is willing to pay the cost to minimize the margin-of-error at every step.
Another approach to keeping yield high (at the possible risk of hurting your brand) is to do what Intel did with their Celeron line. The equivalent for Apple would be a "value iPhone" which would not come with the same set of expectations from customers.