IMO for a place like apple, where they are buying chips in such volumes they define entire production runs anyway and define target design specs for vendors there is probably already a lot of cost they are consuming where adding something like the apple silicon design project is cheaper. The marginal cost of going full apple silicon is lower as a result. Apple has also been very frustrated with their chip vendors, especially intel, in making what they want so I imagine it started as a 'last resort' project after they've exhausted their vendors.

While another smaller place that wanted to do something like apple silicon would have much higher marginal costs because they would need to spend on the things that apple was already spending on implicitly. It became an economy of scale thing.